2. 10. 2023 - Monday Colloquium at Physics Department

Debabrata Deb: Gay-Berne liquid crystal in two dimensions

Thapar Institute of Engineering & Technology, India

It is well established that the melting of two-dimensional (2D) crystals occurs through the unbinding of topological defects, first the unbinding of dislocation pairs and later dissociation into disclinations. However, when the constituent particles are aspherical in shape, topological defects due to their orientational property also appear in the system. Therefore, such orientational defects are expected to also play a role in the melting of 2D liquid crystalline (LC) systems. Here, we report the results of molecular dynamics simulations of a 2D LC system consisting of soft ellipses. The Gay-Berne (GB) potential governs the interaction between the constituent ellipses. We focused our study on the systems’ high-density states as the orientational property remains relevant only up to liquid crystalline nematic phases. We observed an interplay of positional and orientational defects during the melting of the 2D solid phase. We constructed a phase diagram for the dense GB system in the (ρ,T) plane. As an extension of this study, we also looked at the influence of an underlying substrate in the phase behavior of the 2D LC system. We have found substrate-induced freezing, melting, and structural depinning transitions occurring in the system as a function of the substrate periodicity parameter.

[1] Bharti, D. Deb, Melting of dense liquid crystalline Gay-Berne system in two dimensions, under review.

[2] Bharti, D. Deb, Novel smectic phases and orientational order switching in two-dimensional liquid crystalline system, Liq. Crys., 42 1983 (2022).

[3] Bharti, D. Deb, Substrate induced freezing, melting and structural depinning transitions in two-dimensional liquid crystalline system, Phys. Chem. Chem. Phys. 24, 5154 (2022).

14. 11. 2022 - Monday Colloquium at Physics Department

Holger Stark: Insights into dry and wet active matter

Institut für Theoretische Physik, TU Berlin, Germany

Active matter consists of constituents that consume energy in order to self-propel. It can be found in different environments and, in particular, in the microscopic world of micro-organisms and artificial microswimmers, where inertia is negligible. The talk introduces different features of active matter. In dry active matter the frictional coupling to the surrounding is described by a friction coefficient. We use the resulting system of active Brownian particles to discuss active Szilard engines [1] and perform microrheology of an active bath in order to explore its dynamic properties [2].

Microswimmers initiate hydrodynamic flow fields in their aequous environment through which they interact. For this wet active matter, we performed numerical simulations with the method for multi-particle collision dynamics, where we explore the collective dynamics of elongated model microswimmers [3]. Depending on the swimmer type of neutral rods and pusher/pullers, they exhibit different emergent collective dynamics including active turbulence.

[1] P. Malgaretti and H. Stark, Szilard engines and information-based work extraction for active systems, arXiv:2203.13075 [cond-mat.soft].

[2] M. Knezevic, L.E. Aviles Podgurski, and H. Stark, Oscillatory active microrheology of active suspensions, Sci. Rep. 11, 22706 (2021).

[3] A.W. Zantop and H. Stark, Emergent collective dynamics of pusher and puller squirmer rods: Swarming, clustering, and turbulence, Soft Matter 18, 6179 (2022).

7. 4. 2022 - Soft matter physics seminars at Physics Department

Magdalena Lesniewska: Single particle behaviour in a liquid crystal microfluidic channel

Department of Physics, University of Strathclyde, Glasgow, UK

Colloidal particles in liquid crystals exhibit a flow behaviour that is strikingly different from that in isotropic media. On one hand, the emerging topological defects give rise to highly complex, anisotropic elastic forces that the liquid crystal exerts on the colloidal particles. On the other hand, non-linear, flow-dependent forces are also involved, which are impossible to predict analytically. Studies of particles in nematics were therefore generally limited to how the molecules reorder around the solute particles. Here, I will present new results of lattice-Boltzmann simulations of colloid-liquid crystal composite materials in Poiseuille flow. At the single-particle level a new, hydrodynamic and very fast separation effect is observed that is distinct from the well-known Segre-Silberberg effect in isotropic media at low volume fractions. Contrary to the latter, there is no single attractors: where and how particles accumulate depends sensitively on the imposed flow rate and particle size. This new mechanism is attributed to the interplay between the elastic and hydrodynamic forces in combination with flow-alignment of the liquid-crystal and order-flow interactions induced in the defect network.

7. 4. 2022 - Soft matter physics seminars at Physics Department

Irena Drevenšek-Olenik: Magneto Responsive Surfaces for Manipulation of Light and Liquids

University of Ljubljana, Faculty of Mathematics and Physics, and J. Stefan Institute, Ljubljana, Slovenia

Dynamically responsive surfaces are rapidly gaining attention in different areas of modern technology, a well-known example being touch-based interface systems and devices. Among the materials showing promising properties for this kind of applications are composite materials comprised of micrometer-sized ferromagnetic particles embedded in a non-magnetic elastomer matrix that are known as magneto-rheological or magneto-active elastomers (MAEs). I will discuss the results of our

recent investigations of magnetic field-induced modifications of surface topography of MAE, which enable control of various surface properties such as surface reflectivity, wetting and adhesion, by an external magnetic field [1,2]. I will also present different methods for surface micro-structuring of MAE, which lead to promising features for possible use in magnetically tunable diffractive optical elements (DOEs) [3] and magnetically regulable components for application in microfluidic systems [4,5].

[1] G. Glavan, P. Salamon, I.A. Belyaeva, M. Shamonin, I. Drevenšek-Olenik, Journal of Applied Polymer Science 135, 46221 (2018).

[2] G. Glavan, W. Kettl, A. Brunhuber, M. Shamonin, I. Drevenšek-Olenik, Polymers 11, 594 (2019). 

[3] M. Lovšin, D. Brandl, G. Glavan, I.A. Belyayeva, L. Cmok, L. Čoga, M. Kalin, Polymers 13, 4422 (2021).

[4] G. Kravanja, I.A. Belyaeva, L. Hribar, I. Drevenšek-Olenik, M. Jezeršek, M. Shamonin, Adv. Mater. Interf. 8, 2100235 (2021).

[5] G. Kravanja, I.A. Belyaeva, L. Hribar, I. Drevenšek-Olenik, M. Shamonin, M. Jezeršek, Adv. Mater. Technol., 2101045 (2021).

7. 4. 2022 - Soft matter physics seminars at Physics Department

Martina Clairand: Dynamics in active nematic ellipsoidal shells

Gulliver Lab UMR7083, ESPCI, Paris, France

The design of new types of soft structured materials demands for simple ways to control the orientation and the interactions between elementary building elements. Liquid crystals provide good opportunities for this aim due to their potential for assembling microscopic objects via topological defects. Typically, covering a sphere with a thin liquid crystalline layer leads to the formation of a controlled number of topological imperfections which can be tailored to emulate attachment sites for interparticle interactions. Recently, these notions were taken a step further by coupling passive liquid crystals (PLC) with an active material in spherical geometries. When an aqueous mixture of microtubules and kinesin motors is in contact with a PLC, the filaments condense at the interface to develop two- dimensional active nematics punctured by defects that continually move and explore new configurations of space. In this project, we elucidate the dynamic behavior of defects emerging when an active nematic shell develops at the interface with ellipsoidal droplets of a smectic liquid crystal prepared using microfluidic techniques. We show that the ability of the active nematic material to sense the shape and the friction anisotropy imposed by the passive core, gives rise to periodic oscillations, self-regulated by external hydrodynamic forces.

3. 2. 2022 - Soft matter physics seminars at Physics Department

Nerea Sebastián: Ferroelectric nematic phase: RM734 vs DIO

F7 – Complex Matter Department, Jožef Stefan Institute, Ljubljana, Slovenia

The recent discovery of the ferroelectric nematic phase certainly constitutes an important milestone in liquid crystals research. With it, experimental investigation of one of the most searched liquid crystal phases in decades has finally been made possible, with high expectations for future applications. Parallel in time two materials, DIO and RM734, were reported to exhibit additional nematic phase distinct from the classical one. While RM734 shows the phase sequence N-NF, DIO exhibits and additional nematic phase in between, N-M2-NF. In both cases, materials consist of elongated molecules with a large dipole moment at a small angle with the molecular axis. Since then, investigations of both materials run in parallel, showing that the novel nematic phase is indeed ferroelectric, with giant spontaneous polarization and unique dielectric properties. During this talk, based on the comparison of both materials, I will give an overview of our investigations on ferroelectric nematic materials, with special focus on its differentiating dielectric, viscoelastic and optical characteristics with the aim of identifying the key mechanisms involved in the nematic-ferroelectric nematic transition. Boundary conditions, i.e. confining cell treatments, have a high impact on the observed director structures, which will be also covered in this talk.

3. 2. 2022 - Soft matter physics seminars at Physics Department

Alexander Mietke: Odd dynamics of living chiral crystals

Massachusetts Institute of Technology, USA

The emergent dynamics exhibited by collections of living organisms often shows signatures of symmetries that are broken at the single-organism level. At the same time, organism development itself encompasses a well-coordinated sequence of symmetry breaking events that successively transform a single, nearly isotropic cell into an animal with well-defined body axis and various anatomical asymmetries. Combining these key aspects of collective phenomena and embryonic development, we describe here the spontaneous formation of hydrodynamically stabilized active crystals made of hundreds of starfish embryos that gather during early development near fluid surfaces. We describe a minimal hydrodynamic theory that is fully parameterized by experimental measurements of microscopic interactions among embryos. Using this theory, we can quantitatively describe the stability, formation and rotation of crystals and rationalize the emergence of mechanical properties that carry signatures of an odd elastic material. Our work thereby quantitatively connects developmental symmetry breaking events on the single-embryo level with remarkable macroscopic material properties of a novel living chiral crystal system

3. 2. 2022 - Soft matter physics seminars at Physics Department

William Xiaoguang Wang: Liquid Crystal-Infused Porous Surfaces with Molecular Order-Dependent Slipperiness and Cargo Release

Department of Chemical and Biomolecular Engineering, The Ohio State University, USA

The design of open surface microfluidics that enable orthogonal control of liquid mobility and chemical compositions is critical for devising the next generation of microfluidic platforms that will find use in applications across chemical, environmental, and biomedical fields. To achieve these desirable functionalities, extensive studies have demonstrated stimuli-responsive liquid mobility on open fluidic platforms based on either micro/nanoscale topographical surfaces or water-immiscible

liquid-coated surfaces. However, methods of manipulating droplets’ chemical compositions tend to rely upon chemical adsorption directly from the underlying surface, which has been shown to subsequently pin droplets to the surface and render them immobile, as illustrated in Figure A. In this presentation, we report the design of a liquid crystal (LC)-based open surface microfluidic platform that enables the independent manipulation of the mobility and chemical compositions of droplets, as shown in Figure 1B. Specifically, we use porous LC polymeric networks to stabilize thermotropic LC mesogens to overcome the aforementioned issue of water-induced LC dewetting. We find that the mobility of water droplets on LC-based surfaces depends only on the positional order of the LC: water droplets become highly pinned at LC surfaces in the smectic A phase, whereas droplets can freely slide without pinning at LC surfaces in both the nematic and isotropic phases. Moreover, we experimentally and theoretically demonstrate that the mesogenic orientational order of the LC surface plays a pivotal role in the release of chemicals from the LC surface to droplets. Our work provides novel design principles for fabricating anisotropic liquid-based open surface microfluidics that enable promising applications including liquid droplet-based chemical synthesis and medical diagnostics.

27. 1. 2022 - Soft matter physics seminars at Physics Department

Tadej Emeršič: Generic approach to directed self-assembly of polymerizable blue-phase liquid crystals

University of Chicago, Chicago, USA

Blue-phases (BPs) occur in chiral liquid crystals in a narrow temperature window between the chiral nematic and the isotropic phases. BPs exhibit unit cell sizes of the order of 100’s nm, making them soft photonic crystals. Possessing unique structural and optical properties, they have been regarded as one of the most promising candidates for applications in displays, sensors, and lasing. However, polycrystallinity and thermal instability of BPs have still greatly limited their applicability. Chemically patterned surfaces with alternating planar and homeotropic anchoring regions can be used for the epitaxial growth of monocrystalline BPs with a prescribed lattice orientation. In this work, we show that such a simple stripe-like pattern symmetry with different interfacial energy can direct the self-assembly of BP forming mixtures with different chemical compositions into monocrystals. Our experiments together with the support of free energy theoretical calculations propose that the strategy of nano-patterned substrates is generally applicable and may serve as a general platform for nucleation and growth of stable, macroscopic single crystalline area with controlled crystal orientation. Each large single monocrystal of selected BP liquid crystal mixtures reflects the light differently which results in the formation of a colorful spectrum of monocrystals. Moreover, our experiments also demonstrate that the thermal stability of such prepared monocrystals can be dramatically improved by photopolymerization, which freezes and retains the BP structure. This indicates that combining the directed self-assembly and the photopolymerization methods lead to the preparation of thermally stable BP monocrystals of a desire lattice orientation. Engineering and controlling the lattice orientation of the polymerized monocrystals greatly improves the applicability of BPs for technological applications.

27. 1. 2022 - Soft matter physics seminars at Physics Department

Manos Anyfantakis: Cellulose-based, stimuli-responsive, photonic liquid marbles

University of Luxembourg, Luxembourg

Liquid marbles (LMs), liquid drops encapsulated by solvophobic particles, have emerged as a unique platform for transporting the core liquid across various substrates with no leakage [1]. Beyond this, the potential of LMs to serve as ‘mini-reactors’ for making new materials, although realized early [1], has been exploited very little so far, and mostly for “conventional” chemical reactions. We here introduce a new concept, in which a LM acts as a miniature platform for programming the selfassembly of the bio-derived polymer hydroxypropyl cellulose (HPC), into a cholesteric liquid crystal with tailored structural color.

Above a critical HPC concentration, short-pitch cholesteric liquid crystal phases form in aqueous solutions. At sufficiently high concentrations, the cholesteric pitch is reduced to sub-micrometer values and the material shows structural coloration [2]. Bearing this in mind, we prepare LMs of HPC solutions in the biphasic regime, where an isotropic and a cholesteric phase coexist, and by immersing the LMs in an oil phase with poor water miscibility, we slowly extract the desired amount of water. This allows the polymer chains to self-organize into a fully cholesteric structure, the pitch of which can be tuned by programming the final HPC concentration in the LM. This in turn leads to LMs with selective Bragg reflection tailored to be anywhere in the visible. The optical response of the LMs is sensitive to various external perturbations. For instance, LMs respond with color changes that are detected by the unaided eye, to changes in temperature, the presence of toxic chemicals in their environment, and mechanical compression [3]. 

The presented concept demonstrates the advantages of using LMs as a well-controlled experimental platform for inducing, monitoring, and controlling the self-organization of a bio-derived polymer in solution; this may improve our understanding of the liquid crystalline self-assembly of bio-sourced nanomaterials. Furthermore, it highlights the potential of LMs for responsive, soft photonic objectsthat may prove useful for developing sustainably produced, multifunctional sensing and photonic devices.

[1] P. Aussilous, D. Quéré, Nature (2001), 441, 924.

[2] A.P.C. Almeida, et al., Adv. Mater. (2018) 30, 1703655.

[3] M. Anyfantakis, et al., Angew. Chem. Int. Ed. (2020), 59, 19260.

27. 1. 2022 - Soft matter physics seminars at Physics Department

Anže Božič: Mechanical design of apertures and their role in infolding and bursting of pollen grains

Jožef Stefan Institute, Ljubljana, Slovenia

Pollen carries male plant genetic material encapsulated in a hard protective shell containing flexible, soft regions — apertures. This mechanical structure of pollen shells guides their response to changes in the humidity of the environment. These changes can lead both to pollen infolding upon dehydration, which often occurs once pollen becomes exposed to the environment, but also to excessive swelling and even bursting of pollen grains upon hydration. We investigate the folding and swelling pathways of pollen grains by studying elastic deformations of inhomogeneous thin shells. Different pathways are governed by the interplay between the elastic properties of the hard and soft regions of the pollen shell and by the aperture shape, number, and size. In the case of pollen dehydration, we delineate the regions of mechanical parameters of the pollen grain which lead to complete closure of all apertures, thus reducing water loss and presenting evolutionary viable solutions to the infolding problem. On the other hand, we identify and explore a mechanical weakness of the apertures upon pollen hydration, where the apertures are prone to a rapid inflation once the grain swells to a critical extent. This transition leads to the bursting of the grain and the release of its content, shedding light on the inactive part of the mechanical response of pollen grains to hydration once they land on a stigma as well as on bursting of airborne pollen grains during rapid changes in air humidity.

29. 11. 2021 - Monday Colloquium at Physics Department

Assist. Prof. Andrej Košmrlj: Mechanical Instabilities in Growing Biological Systems: Wrinkling and Branching

Princeton University

Morphological shape transformations in biological systems often arise from patterned biochemical processes, which can produce mechanical forces either directly via molecular motors or indirectly via differential growth of connected tissues. The growth mismatch produces internal stresses, which can be released via shape transformations and mechanical instabilities. In this talk I will focus on mechanical instabilities that cause the wrinkling of Vibrio cholerae bacterial biofilms and branching in developing lungs. Bacterial biofilms grown on substrates form wrinkled patterns that can be manipulated by modifying the substrate stiffness. We showed that the wavelength of wrinkles is consistent with the mechanical stability of compressed films on soft substrates. Furthermore, we demonstrated that the spatiotemporal pattern of wrinkles can be predicted by a continuum chemo-mechanical model that incorporates diffusion of nutrients and their uptake by bacteria, growth of the biofilm, surface friction, and the ensuing mechanical stresses and deformations of the biofilm. In the second part, I will discuss the branching morphogenesis of lungs. By combining experiments and modeling we showed that the patterned formation of stiff smooth muscles and their contractions physically sculpt new branches of growing epithelium. I will also comment on how we are going to use these insights to design an optogenetic system to engineer artificial lung organoids.

11. 11. 2021 - Soft matter physics seminar at Physics Department

Andraž Gnidovec: Simulating the packing of spherical ellipses

Faculty of Mathematics and Physics, University of Ljubljana, Slovenia

Packing problems are abundant in nature and are thoroughly researched both experimentally and in numerical models. In particular, packings of anisotropic, elliptical particles emerge in models of liquid crystals, colloids, as well as granular and jammed matter. While most of these studies deal with packings in Euclidean geometries, there are many experimental systems where anisotropically shaped particles are confined to a curved surface, such as systems of colloidal rods trapped at the interface of emulsion droplets, and protein adsorbates on vesicles. In this talk, I will first present an algorithm we developed to determine overlaps between two spherical ellipses based on the solution of an eigenvalue problem. More generally, the proposed method allows us to define a continuous contact function that could also be applied to long-range interactions, however, the use case as a distance metric comes with some caveats. Returning to the main objective of the algorithm, I will demonstrate its efficiency in packing simulations. I will show the random close packing results for a monodispersed system of hard ellipses and present the challenges with quantifying disorder in generated packings. Finally, I will contrast the results with those in Euclidean geometries and discuss the approach towards the large $N$ limit with vanishing surface curvature.

11. 11. 2021 - Soft matter physics seminar at Physics Department

Carl Goodrich: Assembling function with differentiable simulations

Institute of Science and Technology (IST), Austria

Solving inverse problems is a ubiquitous challenge spanning much of science. This is particularly relevant in the world of synthetic self-assembly, where we seek to create new materials by bringing together constituent building blocks whose size, shape, and interactions can be precisely controlled. But what collection of size, shape, and interactions will lead to the assembly of interesting materials with desirable properties? This inverse problem is challenging because even highly simplified models often contain 10s to 100s of parameters when there are more than just a few particle species. I will present a novel numerical approach for tackling this problem that directly connects experimentally relevant model parameters (e.g.~sizes, shapes, interactions) with the effect on emergent material properties. This approach includes techniques borrowed from the machine learning community to differentiate over entire molecular dynamics simulations and other statistical physics calculations. In addition to enabling us to design self-assembled systems with complex properties and behavior, efficient and accurate gradient (and hessian) information presents a qualitatively different way of approaching classical physics simulations, with applications well beyond synthetic self-assembly.

11. 11. 2021 - Soft matter physics seminar at Physics Department

Nuno Araujo: Self-folding Kirigami at the microscale

Universidade de Lisboa Centro de Física Teórica e Computacional Departamento de Física da FCUL, Lisbon, Portugal

Three-dimensional shells can be synthesized from the spontaneous self-folding of two-dimensional templates of interconnected panels, called nets. To design self-folding, one first needs to identify what are the nets that fold into the desired structure. In principle, different nets can fold into the same three-dimensional structure. However, recent experiments and numerical simulations show that the stochastic nature of folding might lead to misfolding and so, the probability for a given net to fold into the desired structure (yield) depends strongly on the topology of the net and experimental conditions. Thus, the focus has been on identifying what are the optimal nets that maximize the yield [1]. But, what about the folding time? For practical applications, it is not only critical to reducing misfolding but also to guarantee that folding occurs in due time. Here, we consider as a prototype the spontaneous folding of a pyramid. We find that the total folding time is a non-monotonic function of the number of faces, with a minimum for five faces. The motion of each face is consistent with a Brownian process and folding occurs through a sequence of irreversible binding events that close edges between pairs of faces [2]. The first edge closing is well-described by a first-passage process in 2D, with a characteristic time that decays with the number of faces. By contrast, the subsequent edge closings are all first-passage processes in 1D and so the time of the last one grows logarithmically with the number of faces. It is the interplay between these two different sets of events that explains the non-monotonic behavior. Implications in the self-folding of more complex structures are discussed.

30. 9. 2021 - Soft matter physics seminar at Physics Department

Iztok Urbančič: Interplay between protein mobility and lipid order in cellular membranes

Laboratory of Biophysics, Condensed Matter Physics Department, Jožef Stefan Institute, Ljubljana, Slovenia

MRC Human Immunology Unit, Weatherall Institute of Molecular Medicine, University of Oxford, UK

The plasma membrane is a vital structure that defines the cellular boundary, controls the exchange of matter with the environment and transduces the signals that determine further cellular actions, such as activation of immune cells. To achieve such remarkably versatile functionality, the membrane is composed of a myriad of protein and lipid species, forming a dense 2D fluid. The efficiency of membrane-hosted processes is further tuned by dynamic nanoscale segregation of its components. However, due to limited spatiotemporal resolution of experimental techniques, the molecular mechanisms and roles of these structures in cellular processes remain largely debated. To disentangle the elusive lipid-protein interactions underlying these membrane heterogeneities, we investigated how externally imposed variations in mobility of membrane proteins affect the local lipid order. To this end, we used spectrally resolved microscopy with polarity-sensitive membrane probes in passive model membranes and in live T cells. Our data also suggest that the induced ordered lipid patches can mediate protein-protein interactions via an interplay between of energy and entropy. The cellular membrane is thus poised to modulate the frequency of protein encounters upon alterations of their mobility, e.g. in ligand binding. This offers new mechanistic insight into the involvement of lipid-mediated interactions in membrane-hosted events, such as immune signalling.

30. 9. 2021 - Soft matter physics seminar at Physics Department

Lisa Tran: Controlling topological defects within achiral and chiral nematics using surface geometry and anchoring

Ornstein Laboratory, Princetonplein 1, 3584 CC Utrecht, The Netherlands

System boundary conditions are significant in determining the arrangement and dynamics of topological defects in liquid crystals. In this talk, I will present two systems that demonstrate the use of boundary geometry to control defects. The first system comprises of an array of holes suspended within a planar nematic cell. Each hole necessitates, topologically, the presence of an accompanying defect. Within an array, the defects within each hole can interact to create intricate defect structures, such as a network of disclination lines that span the system. These structures are determined by the saddle deformations of each hole, the elastic constants of the liquid crystal, and the anchoring of the cell. The second system confines chiral nematics into spherical shells. Adjusting the shell geometry as well as the type and strength of anchoring at the shell surfaces determines the kind and assembly of chiral nematic defects. Outside of equilibrium configurations, I will present recent results where dynamically changing the surface anchoring can undulate the chiral nematic to produce defects, in a manner reminiscent of the classic Helfrich-Hurault instability. Topological defects in liquid crystals can be finely tuned through the command of surface boundary conditions using techniques and principles that could also translate for out-of-equilibrium systems.

16. 9. 2021 - Soft matter physics seminar at Physics Department

Guilhem Poy: Guiding principles of light in frustrated chiral birefringent systems

Department of Physics, Faculty of Mathematics and Physics, University of Ljubljana, Slovenia

In the past 10 years, major developments in the field of soft matter have unlocked the possibility of creating and controlling complex birefringent structures such as Hopf fibrations, knots/links, cholesteric fingers and torons. These structures can be stabilized in anisotropic soft materials—such as liquid-crystal based mixtures—when chiral molecules are added to the system: here, the main role of chirality is to induce meta-stability in the free energy landscape of the sample, thus opening the way to a lot of exciting and novel structures of interest for topological soft matter and optical applications. In this seminar, I will present an overview of the research made during my Marie Curie fellowship at the FMF in Ljubljana. The objective of this project was to establish the guiding principles of light in frustrated chiral liquid crystals based on nonlinear optical effects and light-matter interactions with topological solitons—robust localized structures which cannot be destroyed in a continuous manner. After a general introduction on this soft birefringent system and the different topological objects that can be embedded in it, I will present the theoretical and numerical tools that I developed to study light propagation in inhomogeneous birefringent media. Then, I will show how chirality can boost the nonlinear optical response of liquid crystal systems in the context of optical solitons—self-focused optical beams which can propagate over long distance while preserving their transverse profiles. Finally, I will demonstrate how topological solitons can be used to control the flow of light at the microscopic scale, thus opening new possibilities in this emerging new field of soft photonics.

16. 9. 2021 - Soft matter physics seminar at Physics Department

Julija Zavadlav: Neural network models for molecular simulations based on experimental data

Department of Mechanical Engineering, Technical University of Munich, Munich, Germany

Molecular modeling has become a cornerstone of many disciplines, including soft matter physics. However, the quality of predictions critically depends on the employed model that defines particle interactions. A class of models with tremendous success in recent years are neural network (NN) potentials due to their flexibility and capacity of learning many-body interactions. Traditionally, these models are trained bottom-up on quantum mechanical data. Top-down approaches that learn NN potentials directly from experimental data have received less attention, typically facing numerical and computational challenges when backpropagating through molecular dynamics (MD) simulations. We present the Differentiable Trajectory Reweighting (DiffTRe) method, which bypasses differentiation through the MD simulation for time-independent observables. Leveraging thermodynamic perturbation theory, we avoid exploding gradients and achieve around 2 orders of magnitude speed-up in gradient computation for top-down learning. The effectiveness of DiffTRe is showcased on an atomistic model of diamond and a coarse-grained model of water based on diverse experimental observables including thermodynamic, structural, and mechanical properties. Our approach opens the way to high fidelity molecular models, particularly when bottom-up data is unavailable or insufficiently accurate.

16. 9. 2021 - Soft matter physics seminar at Physics Department

Gašper Tkačik: Avalanches and neural oscillations in a simple model near criticality

Institute of Science and Technology Austria, Am Campus 1, AT-3400 Klosterneuburg, Austria

Brain dynamics display collective phenomena as diverse as neuronal oscillations and avalanches. Oscillations are rhythmic, with fluctuations occurring at a characteristic scale, whereas avalanches are scale-free cascades of neural activity. Here we show that such antithetic features can coexist in a very generic class of adaptive neural networks. In the most simple yet fully microscopic model from this class we make direct contact with human brain resting-state activity recordings via tractable inference of the model's two essential parameters. The inferred model quantitatively captures the dynamics over a broad range of scales, from single sensor fluctuations, collective behaviors of nearly-synchronous extreme events on multiple sensors, to neuronal avalanches unfolding over multiple sensors across multiple timebins. Importantly, the inferred parameters correlate with model-independent signatures of ``closeness to criticality'', suggesting that the coexistence of scale-specific (neural oscillations) and scale-free (neuronal avalanches) dynamics in brain activity occurs close to a non-equilibrium critical point at the onset of self-sustained oscillations.

2. 9. 2021 - Soft matter physics seminar at Physics Department

Simon Čopar: Chiral topological states in flowing achiral nematics

Department of Physics, Faculty of Mathematics and Physics, University of Ljubljana, Slovenia

Nematic liquid crystals are known for supporting a variety of interesting topological structures in equilibrium -- point defects, line defects, nonsingular solitons, periodic textures, and other related features of orientational order. These can be induced and controlled by external electric and optical fields, and most importantly, by confinement. However, subjecting the nematic to flow allows us to achieve additional structures that are not stable under understatic static equilibrium. Flow alignment under shear flow is a well known phenomenon of liquid crystals with appropriate viscosity parameters, and a nematic flow in a thin wide channel that enforces homeotropic alignment at the channel walls, exhibits a first order transition when flow rate is increased above a critical value. But before the flow-alignment transition, the nematic flow is unstable and undergoes a second order symmetry-breaking transition into a chiral state. This state is hard to achieve experimentally, as minor fluctuations in the velocity and orientational profile can trigger the flow-alignment transition, and was only recently first observed and described. In this talk, I will demonstrate the experimental realization of the chiral-pretransitional state achieved by careful pressure control, and present the theoretical description and topological properties of this flow state.

2. 9. 2021 - Soft matter physics seminar at Physics Department

Lou Kondic: Modeling liquid crystal films on nanoscale

Department of Mathematical Sciences and Center for Applied Mathematics and Statistics, New Jersey Institute of Technology, Newark, NJ 07102, USA

This talk will focus on recently developed models and computational techniques for thin films, with focus on nematic liquid crystal films. Models and computations are formulated within the framework of the long wave approach, augmented by the inclusion of nematic-solid interaction forces via disjoining pressure model. Particular aspects that will be discussed involve inclusion of liquid-crystalline nature in the model in a tractable manner. The resulting asymptotic model allows for discussing dewetting type of instabilities of nematic films and in particular the influence of nematic properties on the nature of dewetting. The second part of the talk will build upon the first one by considering explicitly anisotropic nature of the nematic films and the influence which such anisotropy has on dewetting. The analytical techniques are supplemented by large scale GPU based simulations that allow for computing in large domains and for discussion of various instability mechanisms in a fully nonlinear setting.

2. 9. 2021 - Soft matter physics seminar at Physics Department

Anupam Sengupta: Anisotropy in Action: Harnessing emergent dynamics in soft and living matter

Department of Physics and Materials Science, University of Luxembourg, 162 A, Avenue de la Faencerie, L--1511, Luxembourg City, Luxembourg

Anisotropy, the ability of partially ordered materials to respond directionally to a stimulus, is key to the dynamics and functions in living systems. From transporting signalling molecules within cells to transmitting mechanical stresses within growing bacterial colonies, anisotropy underpins the physics of living matter. Outside living systems, liquid crystals (LCs) - formed typically by locally oriented low- molecular-weight rod-like molecules - offer an experimentally tractable system within which the fundamentals of anisotropic interactions can be precisely tuned and studied. In my lab, we harness this setting to, on the one hand, understand the fundamentals of anisotropic cross-talks between bio-relevant fluids and cues, and on the other hand, apply this knowledge to uncover the dynamics of real living systems. During this talk, I will allude to this iterative learn-apply-n-learn strategy to showcase how topological defects emerge within biological systems, regulate their micro-environment, and thereby tune the local material, transport, and mechanical attributes. Analysing living matter as emergent systems where anisotropy mediate evolutionarily well-timed biological functions, introduces a novel, yet general, mechanistic framework applicable for disparate biological systems. I will conclude by discussing how our ability to harness anisotropy -- in action across disparate material fields - could have meaningful implications, from designing novel materials to exploring open questions in biology and translational medicine.

19. 4. 2021 - Monday Colloquium at Physics Department

Dr. Guilhem Poy: Guiding principles of light in frustrated chiral birefringent systems

Faculty of Mathematics and Physics, University of Ljubljana

I will present an overview of the research made during my Marie Curie fellowship, whose goal was to establish the guiding principles of light in frustrated chiral liquid crystals (LC) based on nonlinear optical effects and light-matter interactions with robust topological structures. After an introduction of this soft matter system, I will present the theoretical tools that I developed to study light propagation in inhomogeneous birefringent media. Then, I will show how chirality can boost the nonlinear optical response of LC systems in the context of self-focused beams. Finally, I will demonstrate how topological solitons can be used to control the flow of light at the microscopic scale. Some results to be presented are described in Phys. Rev. X 10, 031042 and in Phys. Rev. Lett. 125, 077801.

9. 12. 2019 - Monday Colloquium at Physics Department

Assoc. Prof. Miha Ravnik: Structures of passive and active nematic defects

Faculty of Mathematics and Physics, University of Ljubljana & Josef Stefan Institute, Ljubljana

Complex –passive or active- nematic fluids are characterised by internal orientational order, which upon tuning or frustration, can exhibit topological defects. Here, we present structures of topological defects in passive and active nematic complex fluids. Specifically, we show in passive nematics how confinement in the form of complex geometry and fractal surfaces can lead to formation of various defect-based nematic profiles, including exhibiting high-elastic multipoles. In active nematics, we show defect profiles in three-dimensional active nematic droplet, also highlighting the role of different surface coupling regimes.

30. 9. 2019 - Monday Colloquium at Physics Department

Prof. Helen F. Gleeson: Auxetic liquid crystal elastomers; a bit of a stretch!

School of Physics and Astronomy, University of Leeds, UK

Materials are said to be auxetic if they have negative Poisson’s Ratio. Liquid crystal elastomers (LCEs) are crosslinked rubbers which include liquid crystalline units that provide functionality, order and anisotropy. LCEs respond at a molecular level, exhibiting shape-responsivity and programmability when actuated with light, heat or mechanical stimulation. We recently discovered the first synthetic material to display molecular auxeticity in an LCE that we designed and synthesised. Molecular auxetics have been a longstanding, unrealised goal for materials scientists with several nano-engineering approaches suggested, so this discovery offers a paradigm shift for materials science and engineering. In fact, the nanoscale functionality behind the phenomenon is still something of a puzzle that we’re unravelling.

25. 9. 2019 - Soft matter physics seminar at Physics Department

Prof. Jun-Ichi Fukuda: Exotic structures and their optical properties of a thin cell of a chiral liquid crystal

Department of Physics, Faculty of Science, Kyushu University, Japan

We show numerically that a chiral liquid crystal (LC) with strong chirality, when confined in a thin cell, exhibits various exotic ordered structures, depending on temperature, cell thickness and the type of surface anchoring. Such structures include a hexagonal lattice of Skyrmions, swirl-like excitations that have been attracting considerable interest in condensed matter physics. We also investigate the optical properties of such structures, especially focusing on their Kossel diagrams and microscope images. Calculation of the response of the LC structure to incident light by directly solving the Maxwell equations enables numerical construction of microscope images and Kossel diagrams. Numerical Kossel diagrams and microscope images agree well with experimental ones, which demonstrate that the structures found in our numerical calculations are indeed realized experimentally.

29. 7. 2019 - Soft matter physics seminar at Physics Department

Dr. Jack Binysh: Knotted fields in soft matter, and how to make them

University of Warwick

Knotted fields are physical fields containing knotted structures - structures which then talk to the field as a whole. Examples abound, but in this talk I will describe two liquid crystal examples which have attracted considerable recent interest: Hopfions in cholesteric cells, and Knotted Disclination lines. I will then talk about how one can construct these textures theoretically, including our recent work on constructions using Maxwell's solid angle function.

27. 6. 2019 - Soft matter physics Colloquium at IJS

Prof. Dr. Shu Yang: Geometry, topology, and liquid crystals: The materials applications

University of Pennsylvania

“Geometry is perhaps the most elementary of the sciences”. Geometry is concerned with the properties of configurations of points, lines, and circles, and topology is concerned with space, dimension, and transformation. We will discuss our journey of discovering the beauty and power of geometry, and applications in a wide variety of materials at all scales. Through designs of geometric surface patterns, here, micropost arrays, we program topological defects, and thus the orientational elasticity in liquid crystal assemblies. When coupling chemical patterning and surface topography, we demonstrate symmetry breaking, allowing for multi-state optical switching. Geometry is materials independent. Taking the knowledge in small molecule liquid crystals, we preprogram arbitrary 3D shapes in 2D sheets of liquid crystal elastomers. By coupling with soft forces, we vertically aligned atomically thin 2D nanosheets of titanium carbide from the MXene family up to 200 μm to store more energy. Geometry is also scale invariant. Lastly, we will show the prospective of taking geometry to create smart fabrics.

8. 4. 2019 - Monday Colloquium at Physics Department

Prof. Dr. Saša Svetina: Physical aspects of the origin of life

Institute of Biophysics, Faculty of Medicine UL, and Jožef Stefan Institute

Most studies of the origin of life concentrate on properties of molecules that constitute the living systems. Here it will be assessed that life could have emerged on the basis of shape behavior of lipid vesicles. This behavior is an emergent property of vesicles because it does not depend on the chemical nature of their membranes. I shall first present an overview of the theory of vesicle shapes. Then I shall describe the process of vesicle self-reproduction and argue that it involves the essential characteristics of the life process. Seminar will end by revealing some common features of the process of vesicle self-reproduction and the contemporary cell cycle.

25. 3. 2019 - Monday Colloquium at Physics Department

Dr. Aviel Chaimovich: Molecular Simulations of Aqueous Assembly

Max Planck Institute of Colloids and Interfaces, Potsdam

I investigate two routes in aqueous assembly. In a colloidal project, we resolve a paradox for hydrophobic association, clarifying that small colloids assemble via an entropic oscillatory force and that large colloids assemble via an enthalpic monotonic force; by systematically tuning an energetic parameter, we also show the natural emergence of hydrophilic association in colloids. In a polymeric project, we examine a bead-spring chain that embodies a coil-crystal transition, and via perturbation theory, we demonstrate that such a polymer characterizes the universal kinetics observed in force spectroscopy. These molecular simulations have much relevance for biological mechanisms.

18. 2. 2019 - Monday Colloquium at Physics Department

prof. Julia M. Yeomans: Topology in Biology

Oxford University

Active materials, such as bacteria, molecular motors and self-propelled colloids, are Nature’s engines. They continuously transform chemical energy from their environment to mechanical work. Dense active matter shows mesoscale turbulence, the emergence of chaotic flow structures characterised by high vorticity and self-propelled topological defects. The ideas of active matter are suggesting new ways of interpreting cell motility and cell division. In particular recent results indicate that active topological defects may help to regulate turnover in epithelial cell layers and contribute to controlling the structure of bacterial colonies.

7. 1. 2019 - Monday Colloquium at Physics Department

doc. dr. S. Čopar: Patterning of liquid crystal textures

Faculty of Mathematics and Physics, University of Ljubljana

The flowing nature of liquid crystals allows self-organization and reconfigurability, and their anisotropy is ideal for making patterns that respond to and modulate light. Controlling the patterns made by liquid crystals is thus a forefront of research, not only for displays, but in basic research as well. I will present various ways of producing and controlling patterns in liquid crystals, including colloidal inclusions, creation of chiral solitons, topographical patterning and photoalignment procedures. I will further discuss the topological nature of produced patterns and present the recently published results of micropatterning with air trapped in etched holes.

6. 12. 2018 - Softmatter physics Colloquium

Prof. dr. Qi-Huo Wei: Orient molecules as you wish

Advanced Materials and Liquid Crystal Institute and Department of Physics, Kent State University, Kent, USA

Liquid crystals consisting of rod-shaped molecules are a remarkable soft matter with extraordinary responsivity to external stimuli. Techniques to control molecular orientations are essential in both making and operating liquid crystal devices that have changed our daily lives completely. Traditional display devices are based on uniform alignments of molecules at substrate surfaces. In this talk, we will present a new photopatterning method for aligning molecules into complex 2D and 3D orientations with sub-micrometer resolutions. This approach relies on so-called plasmonic metamasks to generate designer polarization patterns and photoalignments. We will present the basic principles behind this approach, and a number of intriguing applications enabled by it, including micro-optical devices for laser beam shaping, commanding chaotic motions of bacteria, and creating topological defects with designer structures.

23. 4. 2018 - Colloqium at Physics Department

Dr. Jeffrey Everts: Electric double layers in- and out-of-equilibrium

Faculty of Mathematics and Physics, University of Ljubljana

The electric double layer is of central importance whenever a charged surface and a charged fluid meet. For example, a charged solid in contact with an electrolyte results in a diffuse ion cloud that screens the surface charge density on the solid surface. Other examples are found in colloidal dispersions, proteins, DNA and electrochemical cells. In this talk, the physics of electric double layers and some recent developments will be covered. I will discuss how electric double layers affect the phase behaviour and effective interactions of colloidal dispersions, but also how the shape of the double layer couples to the shape of a colloidal particle. Finally, I will briefly discuss recent work on electric double layers out of equilibrium, where flow-induced electric fields are coupled to charge-regulating, charged walls.

19. 4. 2018 - Condensed Matter Physics Department Seminar

Dr. Masoomeh Hashemi: Nematic colloids in topological environments: the mutual role of topology and geometry

Faculty of Mathematics and Physics, University of Ljubljana

Bestowed by their highly anisotropic, stimuli-responsive constituents and their fluidity, nematic liquid crystals are credited with several promising properties such as orientational ordering, anisotropic elasticity, high responsiveness to external ordering fields, quick switchability and local reconfigurability. The combination of such properties has made a nematic liquid crystal a generating agent for an abundant diversity of topological structures spanning from different forms of defect points and lines to variety of solitonic structures. In this seminar, we present results regarding various geometric shapes of colloidal particles (from simple shapes like spheres and tori to complex shapes such as knots and fractals) and different designs of boundary environments. More generally, we place our focus on the mutual role of geometry and topology of the colloidal and confining surfaces in shaping the equilibrium state of nematic liquid crystals.

22. 3. 2018 - Invited lecture at the Days of Jožef Stefan

Prof. dr. Slobodan Žumer: Frustrated liquid-crystalline ordering as a key to topological soft matter

Faculty of Mathematics and Physics, University of Ljubljana & Jozef Stefan Institute, Ljubljana

Topological soft matter is characterized by stable defects in its orientational ordering. The complexity and stability of defect structures depend on the frustration caused by conflicting effects of chirality, anisotropic elasticity, confinement, and external fields. We will show how simple liquid crystals like nematic, cholesteric, and blue phases form a "topological playground" where, by controlled frustration, we can stabilize point & line singular defects, and non-singular solitonic structures like skyrmions. In contrast to the well-known use of liquid crystals in displays, where defects are undesired objects, topological soft matter opens up new possibilities for applications in photonics, plasmonics, and sensorics.

10. 1. 2018 - Softmatter Seminar

Dr. Guilhem Poy: On the pertinence of the thermomechanical model in the Lehmann rotation of cholesteric and nematic droplets

ENS de Lyon, France

This talk is focused on the Lehmann effect, an out-of-equilibrium effect which couples a temperature gradient with the rotation of the internal texture of liquid crystal droplets in coexistence with the isotropic phase. First, we characterized the thermomechanical couplings of Leslie, Akopyan and Zel'dovich by measuring the rotation velocity of the molecules in two translationally invariant configurations with different orientations, below the cholesteric/isotropic transition. Then, we characterized the texture of the droplets observed in the Lehmann experiment, both using optical observations and numerical simulations. More important, we showed for the first time that it is possible to observe the Lehmann effect in achiral nematic droplets, providing that the internal texture is chiral. We also used a photobleaching experiment to show that there is no visible flow in the vicinity of the droplet, which implies that the texture rotation is due to a local rotation of the molecules -- not to a solid rotation of the droplet. Finally, we proposed a theoretical model of the Lehmann effect based on the thermomechanical coupling of Leslie, Akopyan and Zel’dovich. By applying this model to the numerically computed textures, we fitted the measured rotation velocities and found values for the thermomechanical coupling constants much bigger than those measured below the cholesteric/isotropic transition. This shows that this model is incorrect and that the Leslie paradigm must be abandoned.

18. 12. 2017 - Colloqium at Physics Department

prof dr. Slobodan Žumer: Topological soft matter: from knotted braids to skyrmion lattices

Faculty of Mathematics and Physics, University of Ljubljana & Jozef Stefan Institute, Ljubljana

Stable & metastable point defects, disclination lines, and solitons in the orientational order parameter field characterize frustrated nematogenic complex fluids. In such a topological soft matter, frustrations are a result of conflicting influences of confining geometries, surface anchoring, external fields, chirality and elasticity. Basic concepts and approaches needed to deal with topological soft matter will be given. The synergy of numerical modeling and experiments will be illustrated with two examples: i) nematic colloids including knotted disclination braids and chiral solitonic deformations; ii) skyrmion lattices and individual skyrmions in confined blue phases.

13. 11. 2017 - Colloqium at Physics Department

dr. Marco G. Mazza: The complex life of a microswimmer

Max Planck Institute for Dynamics and Self-Organization, Göttingen, Germany

Active particles convert energy from chemical, biochemical, or other processes into motion. Their collective motion has attracted enormous interest on account of the technological applications of artifcial and biological particles. By combining theoretical arguments, molecular dynamics simulations, and experiments we address the complex life of microswimmers in the presence of confining surfaces and hydrodynamic flow. Specifically, we will discuss motion in the presence of curved interfaces, and effects of flows on the clustering of microswimmers. We will conclude by discussing the role of hydrodynamics on swimmers motion.

24. 7. 2017 - Colloqium at IJS

dr. Dario Corradini: Physical Review: A family of journals serving the physical community. The role of PRX

American Physical Society, Ridge, USA

The central mission of the Physical Review magazine is to serve the physics community by developing high-quality media to publish the best research results in specialized magazines as well as in highly selective magazines with a wider theme. In a changing publishing environment, Physical Review strives to remain a desirable place to report on the latest results in the field of physics. Physical Review X (PRX), despite its youth, is already one of the most eminent and most prominent physical magazines with a wide interdisciplinary circle of readers. In the presentation we will review the family of Physical Review magazines and describe the main characteristics of PRX, then discuss some aspects of the review of manuscripts and the cooperation of authors, reviewers and editors.

22. 3. 2017 - Softmatter Seminar

Prof. Giampaolo D'Alessandro: Multiscale models of nematics with inclusions

University of Southampton, England

Suspension of nanoparticles in liquid crystals have been modelled on a range of scales, from molecular simulations to macroscopic models. The former are computationally expensive and only a few particles can be modelled. The second rely on macroscopic parameters whose values are not determined self-consistently. In this talk I will show how to use homogenisation theory to derive macroscopic models of colloidal suspensions in the limit of weak anchoring between particles and liquid crystals. These models contain no free parameters: the coefficients of the macroscopic equations are fixed by the microscopic interaction between particles and liquid crystals and by the geometry of the problem. In the case of fixed particles, the macroscopic equations for the director alignment contain three key differences with respect to those for a pure liquid crystal: (i) the elastic constants are in general smaller, (ii) there is a forcing term proportional to the anisotropy of the particles and, (iii) the dielectric susceptibility of the system is altered due to the fringe fields created by the particles. In the case of rotating particles, the macroscopic equations describe the particle-nematic interaction in terms of (i) their elastic coupling through surface anchoring, (ii) the screening effect of the particle on the nematic distortions and (iii) the coupling of the particles with the applied electric field. Depending on the strength of the particle-liquid crystal interaction the system may display one or two time scales and non-trivial threshold behaviour.

19. 1. 2017 - Softmatter Seminar

Prof. E. Brasselet: Spin-orbit optomechanics

University of Bordeaux, France

In the recent years, unconventional optomechanical effects attracted a growing interest, for instance the so-called pulling and lateral optical forces. This is however not restricted to translational degrees of freedom and also applies to the rotational degrees of freedom. We will discuss the role of spin-orbit interaction of light in such optomechanical effects that open the topic of spin-orbit optomechanics, and which will be illustrated by recent experiments structured solid-state and their extension to soft matter systems, namely liquid crystals.

26. 10. 2016 - Colloqium at IJS

prof. dr. Francesc Sagués: Active nematics at interfaces

Universitat de Barcelona, Barcelona, Spain

Active liquid crystals are a new class of soft materials that have recently raised a huge interest. In particular, reconstituted suspensions of cytoskeletal filaments and associated motor proteins have proven ideal for quantitative studies of the origin of subcellular organization. Here we refer to the system initially engineered by the group of Z. Dogic (Brandeis University), consisting of bundled microtubules powered by ATP-fueled kinesin motors. We concentrate on two-dimensional preparations showing nematic textures and streaming flows, from largely-organized to seemingly chaotic. We will present results on different scenarios where this active nematics system is conditioned with interfacial fluids. The simplest situation corresponds to prepare them in contact with isotropic oils of different viscosities. From this we can extract a prediction for the as of now unknown shear viscosity of the nematic film. More striking is the situation when the contacting passive fluid is a liquid crystal in its smectic phase. In this latter situation a totally unprecedented strategy of control of the active flows has been recently demonstrated. Other scenarios corresponding to encapsulated active nematics, both in contact with isotropic and anisotropic oils will be briefly presented.

24. 10. 2016 - Colloqium at Physics Department

Dr. Matjaž Humar: Live lasers and biocompatible optical waveguides

Jožef Stefan Institute, Ljubljana

We have for the first time demonstrated a laser completely embedded within a live human cell. This achievement was featured on the cover of Nature Photonics, as well it triggered a huge scientific and media interest. These cellular lasers have been used for tagging of a large number of cells and for very sensitive real time intracellular sensing, including real time force measurements. Entirely biological optical cavities and bioluminescence optical pumping has also been demonstrated. Further, implantable biodegradable optical waveguides have been engineered for medical diagnosis and treatments. Based on biocompatible materials these devices solve the limitations of conventional fibers made of glass or plastics for delivering light into tissues.

17. 10. 2016 - Colloqium at Physics Department

Prof. Irena Drevenšek Olenik: Why one can never get bored of optics - optical properties of periodic structures in liquid crystals

Faculty of Mathematics and Physics, University of Ljubljana and J. Stefan Institute, Ljubljana

Periodic structures in liquid crystals occur either spontaneously or can be generated by external stimuli. They are associated with many interesting optical properties, a well-known example being the saturated colour appearance of cholesteric liquid crystals due to iridescence. In the seminar three examples of appealing optical features recently discovered in such systems will be presented: (i) unclonable colour encodings generated by assemblies of cholesteric liquid crystal shells, (ii) puzzling optical diffraction from polarization gratings holographically recorded in liquid crystalline media, and (iii) generation of optical vortices by liquid crystalline structures formed by out-of-plane alignment method.

13. 10. 2016 - Softmatter Seminar

Dr Jeffrey Everts: Non-touching charged colloidal particles near an oil-water interface

Utrecht Unviersity, Netherlands

In this talk I will give a brief overview on the physics of charged colloidal particles with double layers larger than the particle size. In particular, I will show that the interaction of an oil-dispersed colloidal particle with an oil-water interface is highly tunable from attractive to repulsive, either by varying the sign of the colloidal charge via charge regulation, or by varying the difference in hydrophilicity between the dissolved cations and anions. In addition, we investigate the yet unexplored interplay between the self-regulated colloidal surface charge distribution with the planar double layer across the oil-water interface and the spherical one around the colloid. We explain recent experiments where so-called "non-touching" oil-dispersed colloids were detached from an oil-water interface by the addition of an organic salt to the oil, while the particles were initially being trapped at a finite, but small distance from the interface

3. 10. 2016 - Colloqium at Physics Department

Prof. Oleg Krichevsky: DNA as an Exemplary Polymer

Physics Department, Ben-Gurion University of the Negev, Beer-Sheva, Israel

DNA differs from synthetic polymers by its relatively high stiffness: thermal fluctuations bend DNA at the scale much larger than the diameter of DNA double helix (while most of the synthetic polymers bend at the monomer scale). I will show that in some experimental situations DNA polymers follow the predictions of the simplest polymer theories. I will focus on our recent measurements of the structure of dilute and semi-dilute DNA solutions. We developed a new approach to study DNA structure and dynamics through a combination of fluorescence labeling and fluorescence correlation spectroscopy. We show that DNA behaves as an ideal coil in dilute solutions and as a mean-field polymer in semi-dilute solutions.

29. 6. 2016 - Softmatter Seminar

Prof. Dong Ki Yoon: Molecular arrangement of liquid crystal phases in the confined geometries at micron and nanometre scales

Graduate School of Nanoscience and Technology, AIST, Daejeon, Republic of Korea

The orientation control of liquid crystal (LC) phases is essential for fundamental studies as well as practical applications. Surface treatment and topographic confinement have emerged as two of the most effective tools to control ordering and orientation of various types of LC phases. This talk is intended to give an overview of the LC phases controlled in confined geometries at micron and nanometre scales. In addition, the electric field-driven uniaxial alignment techniques will be introduced and the effective analytical techniques will be shown. Finally, the talk will close with the applications using such confined LC phases.

18. 4. 2016 - Colloqium at Physics Department

Prof. Samo Kralj: Effective Topological Charge Cancellation Mechanism

Faculty of Natural Sciences and Mathematics, University of Maribor, and Jožef Stefan Institute, Ljubljana

Topological defects (TDs) appear almost unavoidably in continuous symmetry-breaking phase transitions. Topological origin makes their key features independent of systems' microscopic details and therefore TDs display many universalities. Because of their strong impact on numerous materials' properties and their significant role in several technological applications, it is of strong interest to find simple and robust mechanisms controlling positioning and local number of TDs. In the lecture I will present our numerical study of TDs within effectively two-dimensional closed soft films exhibiting in-plane orientational ordering. We introduce the Effective Topological Charge Cancellation mechanism that efficiently predicts localized positional assembling tendency of TDs and formation of pairs {defect, antidefect} on curved surfaces and/or presence of relevant "impurities" (e.g. nanoparticles).

25. 3. 2016 - Colloqium at Physics Department

Prof. David Andelman: One Hundred Years of Electrified Interfaces: Past, Present and Future

School of Physics and Astronomy, Tel Aviv University, Israel

The Poisson-Boltzmann theory is a mean-field description of ionic solutions and charge interfaces, and it has been instrumental during the last century to predict charge distributions and interactions between charged macromolecules. While the electrostatic model of charged fluids, on which the Poisson-Boltzmann description rests, and its statistical mechanical consequences have been scrutinized in great detail, much less is understood about its probable shortcomings when dealing with various aspects of real physical, chemical and biological systems. After reviewing the important results of the Poisson-Boltzmann theory, I will discuss several modern extensions and modifications as applied to ions in confined geometries. They include fluctuations and correlations leading to a surprising attraction of like-charged surfaces, the importance of the ion-dipole interaction in aqueous solutions, and finite size of ions and other short-range interactions on ionic profiles and surface tension of electrolyte solutions.

16. 3. 2016 - Softmatter Seminar

Dr. Yoshiaki Uchida: Organic Radical Fluids: Magnetism and Microfluidics

Osaka University, Japan

Liquid crystals are promising materials for for data storage devices and for biomedical tools beyond prominent display uses. The emergence of new materials often induces the development of new applications. This talk will present the properties of organic radical fluids as one of such new materials. These compounds exhibit nonuniform intermolecular magnetic interactions in the LC and liquid phases, which have never been reported in the field of purely organic solid-state molecular magnetism at high temperature. The response of paramagnetic LC and liquid droplets to the permanent magnet also depends on the kind of LC phases. Finally, it will be demonstrated that the microcapsules made from the paramagnetic liquid show not only the magnetically transport phenomena but also antioxidative properties.

7. 3. 2016 - Colloqium at Physics Department

Prof. Randall Kamien: Smectics!

University of Pennsylvania, USA

Smectic liquid crystals are materials that probe the physics of membranes, crystals, and liquids all at the same time. I will describe these phases and show how the mathematics of special relativity, group theory, and topology can be used to understand the strikingly beautiful and precise observations of this phase.

22. 2. 2016 - Colloqium at Physics Department

Prof. Julia Yeomans: Droplets bouncing on superhydrophobic surfaces

University of Oxford, UK

Drops bounce easily on superhydrophobic surfaces because of low friction. We discuss how the design of the surface can affect the bouncing, leading to a droplet leaving the surface shaped as a flattened disc or an extended cylinder. Lattice Boltzmann simulations and simple theories are used to help understand the reasons behind the unusual bouncing pathways.

11. 6. 2015 - Colloqium at Physics Department

Dr. Etienne Brasselet: Liquid crystals as a test-bed for chiral optomechanics

CNRS & Université Bordeaux, France

Forces exerted by light on matter basically results from the absorption, modification or redirection of the linear momentum of light. A well-known example is the optical radiation pressure exerted by light on the interface between two media. Tailoring the optical radiation forces generally consists to engineer the scattering of light. Here we will discuss how the optical angular momentum of light can be used to control optomechanical effects, which relies on the interplay between the chirality of matter and that of light. This will be illustrated by recent developments on helicity-controlled optomechanics of chiral liquid crystals.

29. 5. 2015 - Colloqium at Physics Department

Prof. Siegfried Dietrich: Critical Casimir Forces

Max Planck Institut für Intelligente Systeme & Institut für Theoretische Physik IV,Universität Stuttgart, Germany

Long-ranged correlations in a fluid near its critical point lead to clearly identifiable effective forces acting on confining walls. The corresponding universal scaling functions are discussed for different boundary conditions and geometries. The theoretical predictions are compared with high precision experimental data for He4 and He3/He4 wetting films near the superfluid phase transition as well as with synchrotron scattering data from classical binary liquid mixtures. Direct measurements and applications for colloidal suspensions are discussed.

24. 3. 2015 - Softmatter Seminar

Chloe C. Tartan: In-Situ Structuring of Polymer Networks and Defects in Chiral Liquid Crystals for High Speed Photonics and Laser Devices

University of Oxford, UK

In this presentation, we demonstrate how the formation of discrete polymer walls can be used to induce a uniform lying helix (ULH) alignment in a short pitch chiral nematic liquid crystal whereby the axis of the helix lies in the plane of the device. Under the application of an electric field perpendicular to the helix axis, an in-plane tilt is induced in the optical axis, which couples to the flexoelectric coefficients. The linear regime of the chiral flexo-electro-optic effect permits fast changes in the optical state leading to microsecond range response times in liquid crystals. Unfortunately, the ULH alignment becomes unstable when confined between spatially uniform aligning surfaces. Here we use a high resolution multiphoton absorption laser scanning lithography technique to form polymer walls of various dimensions and orientations relative to the rubbing of the alignment layersresulting in a spontaneous ULH alignment.

23. 3. 2015 - Colloqium at Physics Department

Prof. Stephen Morris: Templated Chiral Liquid Crystalline Structures for Lasers and Elastomers

University of Oxford, UK

In this presentation, we focus on the developments of laser devices based upon chiral liquid crystalline structures and explore methodologies for wavelength tuning and multi-coloured arrays. Moreover, we show how the chiral nematic and blues phases may be templated using polymer scaffolds thereby leading to extraordinarily wide temperature ranges in the latter case as well as potentially new electro-optic mechanisms. Finally, we describe how these materials can be used to produce elastomers based upon blue phase I.

23. 2. 2015 - Softmatter Seminar

Dr Apala Majumdar: Multistability in planar liquid crystal wells

University of Bath, United Kingdom

We numerically model a multistable liquid crystal device reported by Tsakonas et. al in 2007. This device comprises a periodic array of micron-scale square or rectangular shallow wells filled with nematic liquid crystal material and the well surfaces are treated to induce planar boundary conditions. We model the nematic well configurations within the celebrated Landau-de Gennes theory. We work with two key parameters in the problem: the well cross-sectional length and the surface anchoring strength. For small nano-scale wells, we find a new biaxial order reconstruction pattern. This two-dimensional order reconstruction pattern has not been previously reported in the literature and can be interpreted as a novel defect structure in nano-scale systems.

For large micron-scale wells and relatively strong anchoring, we find six distinct stable states, two diagonal states and four rotated states and study the multiplicity of solutions as a function of the anchoring strength. We compute the transition pathways between distinct stable equilibria, the optimal transition pathways and the energy landscape of these micron-scale wells. There are multiple transition pathways between stable equilibria, mediated by multiple defects in some examples, and the transition pathways are sensitive to the anchoring strength in the weak anchoring regime. This is joint work with Chong Luo, Radek Erban, Samo Kralj and Halim Kusumaatmaja.

15. 10. 2014 - Softmatter Seminar

Masoomeh Hashemi: Nonspherical Colloidal Particles in Nematic Liquid Crystals: Role of geometrical parameters

Department of physics, Sharif University of Technology, Tehran, Iran

Since the recognition of a novel class of long range intercolloidal interactions mediated by the elasticity of the nematic liquid crystal host, extensive research has been devoted to the role of particle geometry in uniform nematic media. Defect structures specifically depend on the geometry of the particles. Therefore, by carefully choosing the geometrical shapes of the particles one can be able to design defect structures with pre-determined architecture in a liquid crystal colloidal system. Finite element methods are powerful tools in dealing with colloidal surfaces in liquid crystals. By employing a finite element method for numerical minimization of the Landau-de Gennes free energy we study the equilibrium orientation and defect structures of a circular cylindrical particle with flat ends under homeotropic anchoring condition in a uniform nematic medium. Different aspect ratios of this colloidal geometry from thin discotic to long rod-like shapes and several colloidal length scales ranging from mesoscale to nanoscale are investigated. We show that the equilibrium state of this colloidal geometry is sensitive to the two geometrical parameters: aspect ratio and length scale of the particle.

20. 10. 2014 - Colloqium at Physics Department

Prof. Zvonimir Dogic: Active nematics on flat and spherical surfaces

Martin A. Fisher School of Physics, Brandeis University, Massachusetts, USA

The laws of equilibrium statistical mechanics impose severe constraints on the properties of conventional materials assembled from inanimate building blocks. Consequently, such materials cannot exhibit spontaneous motion or perform macroscopic work; i.e., a fluid in a beaker remains quiescent unless driven by external forces. Inspired by biological phenomena such as Drosophila cytoplasmic streaming our aim is to develop a new category of materials assembled from animate, energy-consuming building blocks. Starting from a few well-characterized biochemical components we assemble far-from-equilibrium analogs of conventional liquid crystals. Released from the constraints of equilibrium, this internally driven polymeric material exhibits a host of highly-sought after properties including appearance of steady-state streaming flows that are accompanied by the spontaneous unbinding and annihilations of motile defects as well as appearance and subsequent self-healing of fracture lines. Active liquid crystals can serve as a platform for developing novel material applications, testing fundamental theoretical models of far-from-equilibrium active matter and potentially shedding light on self-organization in living cells.

15. 10. 2014 - Softmatter Seminar

Dr. Jun-Ichi Fukuda: A zigzag defect structure in a nematic liquid crystal and its utility as a chirality sensor

National Institute of Advanced Industrial Science and Technology (AIST), Japan

I show that, when confined in a microwrinkle groove, a nematic liquid crystal accommodates a topological line defect (disclination) of zigzag form. The formation of a zigzag disclination can be attributed to the frustration between the normal alignment at the liquid crystal/air interface and the planar anchoring at the groove surface, together with the elastic constant for twist distortions smaller than those for other distortions. I further show that this zigzag disclination becomes asymmetric (that is, "zigs" become longer than "zags") when a chiral gas is blown onto it. It responds to various chiral gas materials, and could be used as a simple sensor detecting the chirality of a gas sample. This work was done in collaboration with experimental colleagues Dr. Takuya Ohzono and Dr. Takahiro Yamamoto both from NRI, AIST, Tsukuba.

7. 10. 2014 - Softmatter Seminar

Anja Humpert: Molecular dynamics simulations of defects around nanoparticles in nematic liquid crystals

Department of Physics, University of Warwick, Coventry, UK

We aim to study topological defects around nanoparticles in liquid crystals. Large-scale molecular dynamics simulations are used to obtain a better understanding of their molecular-level behaviour. Spherical nanoparticles of different sizes inserted into a nematic were simulated using the Gay-Berne (GB) potential. Around the nanoparticles the liquid crystal molecules are frustrated. This is due to the competition between aligning along the main direction of the liquid crystal molecules, called the director, and the anchoring conditions at the nanoparticles surface. For homeotropic, normal, anchoring, Saturn ring defects were found, while for planar anchoring, Boojum defects were observed, which is in excellent agreement with Laundau-de Gennes theory (LdG).

We are also investigating the Satellite defect, which is often found to be stable for large colloids. This defect has a dipolar structure whereas the Saturn ring has a quadrupolar structure. Molecular simulations of pairs of colloids approaching each other were carried out. At small particle separations the defect loops around each colloid start to bend and at very close distances entangled defect structures were observed.

6. 10. 2014 - Colloqium at Physics Department

Prof. Achille Giacometti: Unconventional Phases in Helical Particles and Janus fluids

Department of molecular sciences and nanosystems, Universita' Ca' Foscari, Venezia, Italia

This talk has two parts. In the first part I will discuss the self-assembly properties of a fluid of helical particles modelled as chains of fused hard spheres arranged in a helicoidal fashion and interacting only sterically. I will show how a rich polymorphism is found, with strong departures from the spherocylinder phase diagram. Remarkable is the presence of a special chiral nematic phase with screw-like order, where helices are well aligned along the director and their C2 symmetry axes spiral around this direction with periodicity equal to the particle pitch. Screw-like ordering was observed in colloidal helical flagella. We have fully characterized this phase and show that it is a general feature in the phase diagram of helical particles. In the second part of the talk, I will discuss another uncoventional phase diagram occurring during a self-assembly process of Janus spherical colloids. The phase diagram includes a colloidal-poor (gas) colloidal-rich (liquid) phase separation, which is progressively suppressed by the insurgence of micelles. Finally, I will discuss recent developments obtained when extending the Janus paradigm to an anisotropic particle, that is a Janus “dumbbell”.

11. 6. 2014 - Colloqium at Physics Department

Prof. dr. Kurt Kremer: Adaptive Resolution Simulations for Soft Matter: Applications and New Developments

Max Planck Institute for Polymer Research, Mainz, Germany

The relation between atomistic structure, architecture, molecular weightand material properties is of basic concern of modern soft matterscience. A typical additional focus is on surface interface aspects orthe relation between structure and function in nanoscopic molecularassemblies. Here computer simulations on different levels of resolutionplay an increasingly important role. To progress further adaptiveschemes are being developed, which allow for a free exchange ofparticles (atoms, molecules) between the different levels of resolution. The extension to open systems MD (grand canonical MD) as well as recent Hamiltonian based molecular dynamics and Monte Carlo adaptive resolution methods will be explained. Typical examples include the solvation of polymers in mixed solvents, first approaches to connect particle based simulations to continuum as well as classical quantum adaptive resolution simulations.

22. 5. 2014 - Colloqium at Physics Department

Dr. Myfanwy Evans: Entanglement from hyperbolic striping of triply-periodic minimal surfaces

Theoretical Physics, Erlangen-Nurnberg University, Germany

High symmetry dense packings of trees and lines in the two-dimensional hyperbolic plane can be projected to triply-periodic minimal surfaces. The resulting three-dimensional structures are complex and entangled. A particular family of these patterns on the Gyroid can be used to describe the self assembly of a set of hierarchical and chiral multicontinuous network structures from Y-shaped ABC and ABD three-miktoarm star terpolymers, formed during numerical simulations. In this talk, I will discuss the construction and characterisation of these complex entangled structures alongside applications from structural chemistry to skin swelling.

15. 5. 2014 - Colloqium at Physics Department

Dr. Davide Marenduzzo: Modelling DNA organisation by DNA-binding proteins

School of Physics & Astronomy, University of Edinburgh, UK

DNA in living cells of both bacteria and eukaryotes associates with proteins that continuously bind and dissociate. Some proteins affect local structure (such as histones and histone-like proteins), whereas others act globally to compact whole chromosomal segments (such as CCCTC-binding factors). Brownian dynamics simulations are used here to model both DNA and DNA-binding proteins. In the absence of any explicit interaction between proteins, or between templates, we find that binding of protein which are able to bridge the DNA, spontaneously induces local DNA compaction and protein aggregation. Small proteins interacting with naked DNA form into rows [as on binding of the bacterial histone-like nucleoid-structuring protein (H-NS)], and large proteins into quasi-spherical aggregates (as on nanoparticle binding). We also explore how these ideas apply to the self-assembly, or reconstitution,of chromatin, which can be made in vitro from DNA and histone octamers, provided that appropriate experimental protocols are followed accurately.

18. 2. 2014 - Colloqium at Physics Department

Prof. Primož Ziherl: Mosaic two-lengthscale quasicrystals

Faculty of Mathematics and Physics, University of Ljubljana and Jozef Stefan Institute, Ljubljana

Over the past decade,quasicrystalline order has been observed in many soft-matter systems: in dendritic micelles, in star and tetra block terpolymer melts and in diblock copolymer and surfactant micelles. The formation of quasicrystal from such a broad range of ‘soft' macromolecular micelles suggests that they assemble by a generic mechanism rather than being dependent on the specific chemistry of each system. Indeed, micellar softness has been postulated and shown to lead to quasicrystalline order. We theoretically explore this link by studying two-dimensional hard disks decorated with steplike square-shoulder repulsion that mimics, for example, the soft alkyl shell around the aromatic core in dendritic micelles.We find a family of quasicrystals with 10-, 12-, 18- and 24-fold bond orientational order which originate from mosaics of equilateral and isosceles triangles formed by particles arranged core-to-core and shoulder-to-shoulder. The pair interaction responsible for these phases highlights the role of local packing geometry in generating quasicrystallinity in soft matter, complementing the principles that lead to quasicrystal formation in hard tetrahedra. Based on simple interparticle potentials, quasicrystalline mosaics may well find use in diverse applications ranging from improved image reproduction to advanced photonics materials.

13. 2. 2014 - Colloqium at Physics Department

Prof. Martin Čopič: Ferromagnetic Liquid Crystal

Faculty of Mathematics and Physics, University of Ljubljana & Institut Jožef Stefan, Slovenia

43 years ago Pierre de Gennes hypothesized that a suspension of magnetic particles in nematic liquid crystal can have ferromagnetic properties. Experiments with elongated particles were unsuccessful as either the particles aggregated or the system remained paramagnetic. Nanosized platelets of barium hexaferrite, recently synthesized by Darja Lesjak and Miha Drofenik, order due to elastic forces mediated by the liquid crystal in such a way that magnetic dipolar interaction causes the suspension to become ferromagnetic. Characteristic magnetizing curve is observed, and domains can be switched by external magnetic field. The system shows strong optical response to small magnetic fields. The relaxation rate of the nematic fluctuations, which are coupled to the magnetization, depends linearly on the applied field. The system is well described by a simple free energy.

10. 2. 2014 - Softmatter Seminar

Dr. Jun-ichi Fukuda: Formation of a zigzag defect structure in a liquid crystal confined within a microwrinkle groove

National Institute of Advanced Industrial Science and Technology (AIST), Japan

We show that, when a nematic liquid crystal is confined in a microwrinkle groove of sinusoidal form, it exhibits a line defect of orientational order (disclination line) of a zigzag form, instead of a straight one that can be imagined intuitively. This is attributed to the hybrid anchoring conditions (normal anchoring at the liquid crystal/air interface, and fixed planar anchoring perpendicular to the groove direction at the groove surface), and the preference for twist distortions over splay and bend distortions in usual nematic liquid crystals of rod-like form. We will also discuss the transformation of this zigzag line defect due to temperature change, and the relation of the topological charge of the defect and the sign of the curvature of the groove.

16. 1. 2014 - Softmatter Seminar

Dr. Jun-ichi Fukuda: Optical properties of exotic defect structures in a strongly confined cholesteric blue phase liquid crystal

National Institute of Advanced Industrial Science and Technology (AIST), Japan

Some of highly chiral liquid crystals exhibit cholesteric blue phases, known as exotic three-dimensional ordered phases made up of an intricate network of topological line defects of orientational order (disclination lines), and so-called double-twist cylinders. In our previous studies, we showed that when a chiral liquid crystal exhibiting cholesteric blue phases in the bulk is confined between two parallel substrates, one can observe various defect structures not found in the bulk, including a hexagonal lattice of Skyrmion excitations, a regular parallel array of disclination lines of double-helix form, and an array of four-arm junctions of disclination lines as seen in bulk blue phase II.

In this work, we investigate the optical properties of these exotic defect structures exhibited by a chiral liquid crystal. To be more specific, we calculate the profiles of the reflected and transmitted light when a monochromatic light is incident normally onto the liquid crystal. Our numerical calculations are based on plane-wave expansions. We particularly focus on the reflected light and show that the intensity profiles depend crucially on the structure of the liquid crystal, and the wavelength and the polarization of incident light. Our exotic defect structures found in a thin cell of a chiral liquid crystal could be used as tunable optical gratings and we hope our present study will elucidate how they work.

17. 9. 2013 - Softmatter Seminar

Prof. Douglas Cleaver: Molecular Simulation of Complex Self-Assembly

Sheffield Hallam University

As well as yielding conventional phases, molecular simulations of generic amphiphilic and chromonic systems can also be focused towards more complex self-assembly. Particularly rich behaviour is to be found in regimes where both entropic and enthalpic contributions are significant. In particular, these regimes are associated with mesophases and structures with characteristic lengths which are clearly different from those of the assembling particles. The properties of the structures formed are naturally described in terms of these emergent length-scales.

Growth kinetics and the relative accessibility of competing pathways play important roles in the development of such structures. Detailed post-hoc analysis can be used to gain an understanding of these kinetics. Further, within an appropriate hierarchical self-assembly process, such understanding can be exploited to control the final outcome.

17. 9. 2013 - Softmatter Seminar

Prof. Claudio Zannoni: Predictive Simulations of Liquid Crystals. Where are we now?

University of Bologna

Computer simulations of organic functional materials have progressed a great deal in the last decade, pushed also by the amazing increase of some three orders of magnitude in supercomputer resources. This has allowed to effectively add atomistic approaches to the current arsenal of modelling and simulation tools, making it possible to predict physical properties such as transition temperatures, order parameters and molecular organizations in the bulk and close to interfaces. In the talk we plan to show a number of recent examples of atomistic molecular dynamics simulations of liquid crystalline systems, trying to assess the current state of the art and some potential developments.

23. 5. 2013 - Softmatter Seminar

Daniel A. Beller: Modeling the assembly of colloids with sharp features in nematic liquid crystals

University of Pennsylvania

The self-assembly of colloids in nematic liquid crystals via topological defects has been extensively studied for spherical particles, and some investigations have considered the role of particle anisotropy. Recent advances in microfabrication techniques have led to homeotropic- aligning colloids with more complex shapes, such as cylinders and “microbullets”, which contain sharp corners. We use Landau-de Gennes numerical modeling to investigate the nematic director field and defect structure around such colloids in a thin-cell geometry, and we determine the colloids' equilibrium alignment directions and effective pair interaction potentials as a function of a few relevant shape parameters. We find that small-scale features of complex colloids have significant effects on defect structure and colloidal assembly in ways not captured by far-field approximations. This raises the possibility of selecting self-assembled colloidal structures by tuning particle shape.

20. 5. 2013 - Colloqium at Physics Department

Dr. Ivan I. Smalyukh: Defect "carving" in liquid crystals using light and colloids

Department of Physics and Renewable & Sustainable Energy Institute, University of Colorado, Boulder, USA

Condensed matter systems with ground-state arrays of defects range from the Abrikosov phases in superconductors, to various blue phases and twist grain boundary phases in liquid crystals, and to skyrmion lattices in chiral ferromagnets. In nematic and chiral nematic liquid crystals, which are true fluids with long-range orientational ordering of constituent anisotropic molecules, point and line defects spontaneously occur as a result of symmetry-breaking phase transitions or due to flow, but they typically annihilate with time and cannot be controlled. This lecture will discuss optically patterned and self-assembled two-dimensional arrays of long-term stable point defects and disclination loops bound together by elastic energy-minimizing twisted director structures. In the periodic lattices of defects, we introduce various dislocations (i.e., defects in positional ordering of defects) and use them to generate optical vortices in diffracted laser beams. The lecture will conclude with a discussion of how these findings bridge the studies of defects in condensed matter physics and optics and may enable applications in data storage, singular optics, displays, electro-optic devices, and diffraction gratings.

13. 5. 2013 - Colloqium at Physics Department

Prof. Christos N. Likos: Hierarchical self-assembly of associating soft patchy particles

Faculty of Physics, University of Vienna

We consider star-­shaped block copolymers (SBC’s) with a sopvophilic core and a solvophobic tail on each arm as novel, model particles which self-­assemble into soft patchy colloids of tunable patchiness at the single-­molecule level and then into a variety of disordered and ordered structures at finite concentrations. In particular, we use the arm number (functionality), f, as well as the fraction of attractive end-monomers, α, as control parameters of the the macromolecules, which give rise to various scenarios of self-­assembly. By employing a variety of computational and theoretical tools, ranging from the microscopic to the mesoscopic, coarse-­grained level in a systematic fashion, we investigate the crossover between the formation of microstructure vs. macroscopic phase separation, as well as the formation of gels and networks in the system. We finally show that SBC’s can be used as building blocks for the fabrication of open crystal structures, such as the diamond or the simple-­cubic lattice, taking advantage of the strong correlation between single-­particle patchiness and lattice coordination at finite densities.

22. 4. 2013 - Colloqium at Physics Department

Dr. Gareth Alexander: Geometry and topology in soft matter: elucidating the hidden symmetries in focal conics

Department of Physics & Centre for Complexity Science, University of Warwick, Coventry, UK

The combination of long-range order and fluid softness in liquid crystals goes hand-in-hand with intricate and exquisite textures of a distinctive geometric, or topological, nature. One of the most remarkable of these are focal conics -- the hallmark of layer order in smectic materials. These are beautiful and geometrically precise conic sections that are observed to occur in pairs of perfect confocal partners and with adjacent domains often exhibiting a more widespread level of geometric organisation, forming intricate networks and trellises, grain boundaries, and Apollonian packings.

I will survey the history of focal conics from the pioneering work of Friedel and Grandjean, who elucidated their pristine geometry, before describing our own contributions that reveal an enhanced conformal symmetry of focal conic domains coming through an analogy with light-like surfaces in special relativity. I will use this perspective to shed light upon several classic focal conic textures, including the concentric cyclides of Dupin, and in doing so illustrate the uses of geometric and topological constructions in understanding the properties of soft materials.

11. 4. 2013 - Softmatter Seminar

Tom Machon: Nematics, Knots and Non-Orientable Surfaces

University of Warwick, Coventry, UK

We extend existing work on spherical and toroidal colloids to describe the topological implications of colloids representing non-orientable surfaces and show that, by controlling colloid geometry and topology, metastable disclinations in the shape of torus knots and links of type (p, 2) can be built around multiply-twisted Mobius strip colloids. These knotted configurations bear a striking resemblance to torons, suggesting a route for experimental realisation.

18. 3. 2013 - Colloqium at Physics Department

Dr. Pietro Cicuta: Conditions of hydrodynamic synchronization in models of beating cilia

Cavendish Laboratory, University of Cambridge, Cambridge, UK

Motile cilia are highly conserved structures in the evolution of organisms, generating the transport of fluid by periodic beating, through remarkably organized behavior in space and time. It is not known how these spatiotemporal patterns emerge and what sets their properties. Individual cilia are nonequilibrium systems with many degrees of freedom. However, their description can be represented by simpler effective force laws that drive oscillations, and paralleled with nonlinear phase oscillators studied in physics.

Here I will describe synthetics model phase oscillators, where colloidal particles are driven by optical traps. The complex structural details of the cilia are coarse-grained into the details of how the colloidal particles are driven. We explore experimentally two types of colloidal model, finding in each case the conditions for optimal coupling. The applicability of this approach to biological data is illustrated by successfully mapping the behavior of cilia in the alga Chlamydomonas onto one of the the coarse-grained models.

25. 2. 2013 - Colloqium at Physics Department

Dr. Dušan Babič: Growing Cells on Colloidal Scaffolds

Fakulteta za matematiko in fiziko, Univerza v Ljubljani

One of the struggles in regenerative medicine is a development of methods for successful growing of an organ tissue (i.e. skin, bone cartilage etc.) which is used in therapy to replace and/or promote healing of the damaged one. A correct tissue growth and formation require an appropriate three-dimensional substrate or scaffold. This is usually a foam like structure composed of interconnected patches of a base material. The scaffold needs to be able to support cells and promote their growth. In Laboratory for Experimental Soft Matter we have developed a flexible method for producing such substrates with super-paramagnetic colloids. The method provides a control over the general structure of the scaffold such as its orientation, branching and void size distribution.

In my talk I will present the method and underlying physical mechanisms of colloidal self assembly. Further I will show the results of experiments in which living cells were successfully grown on so produced colloidal scaffolds. We demonstrated that the cells successfully adhere to the scaffolding structure which is able to support their weight and retain its structural integrity. In our simple growth chamber cells survived and proliferated for several days which is a prerequisite for any successful tissue growth.

22. 10. 2012 - Colloqium at Physics Department

Dr. Matej Praprotnik: Coupling atomistic and continuum hydrodynamics

National Institute of Chemistry Ljubljana & Faculty of Mathematics and Physics, University of Ljubljana

Hybrid methods that couple atomistic and continuum descriptions of liquids are usually based either on a flux-exchange or a state-exchange coupling. The flux-exchange approaches are designed to explicitly conserve mass and momentum fluxes across the atomistic/continuum boundary. The state-exchange approaches, on the other hand, impose in an alternating way the local velocities and densities from both descriptions to be equal at the overlapping domain until the steady state is reached. In this talk, I will present two hybrid approaches, each from the different class. The first method, based on the flux-exchange, concurrently couples atomistic, mesoscopic, and continuum models of the liquid. This triple-scale approach enables the insertion of large molecules into the atomistic domain via a mesoscopic region and it allows for molecular simulations either in the grand-canonical ensemble or under non-equilibrium conditions. The applicability of the second scheme, which bridges flux-based and Schwarz domain decomposition algorithms, will be demonstrated on a three-dimensional multiscale flow simulation of water past a fullerene.

8. 10. 2012 - Colloqium at Physics Department

Dr. Gregor Skačej: Molecular modeling of liquid crystal elastomers

Fakulteta za matematiko in fiziko, Univerza v Ljubljani

Liquid crystal elastomers (LCE) are soft functional materials consisting of weakly crosslinked polymer networks with embedded liquid crystalline (mesogenic) molecules. Consequently, LCE are characterized by a pronounced coupling between macroscopic strain and orientational mesogenic order. As the latter can be controlled by external stimuli such as temperature, electric field, or ultraviolet light, LCE have great potential for application as sensors and actuators [1].In this talk large-scale molecular simulations of swollen main-chain LCE will be presented. The simulated experiments include temperature scans, stress-strain runs, and the application of an external electric field. So far, isostress Monte Carlo simulations of LCE have been capable of reproducing isotropic, nematic and smectic phases, as well as a stress-induced isotropic-to-nematic transition [2]. Moreover, a transversal electric field has been seen to induce nematic director rotation resulting in orientational stripe domains [3].The rather extensive simulation output can also be used to predict typical experimental observables, such as LCE sample dimensions, specific heat, deuterium magnetic resonance spectra, and scattered X-ray patterns.[1] M. Warner and E. M. Terentjev, Liquid Crystal Elastomers (Oxford University Press, Oxford 2003)[2] G. Skačej and C. Zannoni, Softmatter 7, 9983 (2011)[3] G. Skačej and C. Zannoni, Proc. Natl. Acad. Sci. USA 109, 10193 (2012)

30. 8. 2012 - Colloqium at Physics Department

Prof. Randall D. Kamien: Burgers, Lorentz, and Smectics

University of Pennsylvania, USA

Smectic liquid crystals represent the simplest form of broken translational order. The textures seen when defects occur, be they dislocations or disclinations, are the usual means for identifying these phases. I will discuss new methods for visualizing and generating ground state configurations with prescribed topology.

28. 8. 2012 - Colloqium at Physics Department

Prof. Hajime Tanaka: Commonality between viscoelastic phase separation and mechanical fracture

University of Tokyo, Japan

Phase separation is one of the most fundamental phenomena that create spatially inhomogeneous patterns in materials and nature. It has so far been classified into three types: (i) solid, (ii) fluid, and (iii) viscoelastic phase separation. Here we report another phase-separation behaviour accompanying fracture, which is observed under a sufficiently deep quench in polymer solutions. Surprisingly, fracture becomes a dominant coarsening process of the phase separation. Under a deep quench, a transient gel is formed by strong attractive interactions between polymers. The connectivity of the polymer network acts against phase separation and produces the internal stress field. When this stress field exceeds the mechanical stability limit of the transient gel, mechanical fracture takes place: fracture phase separation. The behaviour of viscoelastic and fracture phase separation originates from a strong coupling between composition and deformation field. We demonstrate that the same type of coupling between density and deformation field leads to cavitation of fluid under shear and mechanical fracture of glassy liquid and solid under deformation. The key common concept is "dynamic asymmetry". We discuss a common physics underlying these apparently unrelated phenomena and a selection principle of the kinetic pathway of pattern evolution. For example, the only difference between phase separation and fracture may stem from whether deformation is produced internally by phase separation itself or externally by loading.

18. 6. 2012 - Softmatter Seminar

Prof. Ingo Dierking: Nanoparticles and Nanostructures in Liquid Crystals

School of Physics and Astronomy, University of Manchester, UK

We introduce the combinations of three forms of soft matter, namely liquid crystals, polymers and colloids, by presenting the recent results on various systems. These are

(i) liquid crystal-nanotube dispersions,

(ii) the stabilization of frustrated defect phases (Blue Phases) through the addition of nano-spheres, and

(iii) polymer stabilization of liquid crystal phases and defects.

Combinations of all three forms of soft matter will also be discussed. The physics of these systems will be introduced, and applicational concepts outlined.

12. 4. 2012 - Colloqium at Physics Department

Dr. Miha Ravnik: Complex nematic fluids: colloids, activity, and micro-flow

Faculty of Mathematics and Physics, University of Ljubljana, Slovenia

Complex fluids with internal structure are materials of interesting, generally strongly anisotropic, physical phenomena. Active complex fluids, like swimmers, show collective rheology with internal order, liquid crystalline fluids give rise to complex optic and photonic properties, and colloids can self-assemble into micro and nanostructures with tuneable material characteristics. Here, I will present our recent contributions in complex nematic fluids, focusing on (i) the assembly of three-dimensional colloidal crystals in (passive) nematic liquid crystals, and (ii) on the active flow of a bio-complex fluid -the active nematic - in confinement. I will present relaxation method and the hybrid Lattice Boltzmann methods, as the two main tools that we employ for modelling of complex fluids. Specifically, the analogy between the dynamics of active nematic fluids and (passive) nematic liquid crystals will be highlighted.

22. 3. 2012 - Softmatter Seminar

Anupam Sengupta: Guided and spatio-selective transport via anisotropic fluid flows

Max Planck Institute for Dynamics and Self Organization, Göttingen, Germany

Microscale flow of anisotropic fluids (nematic liquid crystal) offers a novel approach in the realm of transport phenomena. Colloidal particles, either isolated or self-assembled chains, are transported through the bulk in defined spatial orientations or guided through flow induced topological defect structures. The transport of colloidal entities can be additionally tuned using an external field or by means of smart microfluidic architecture. In addition to flow parameters and channel dimensions and geometry, significant influence of the equilibrium molecular orientation of the liquid crystal on the channel walls (surface anchoring) has been observed. Surface anchoring of the molecules affects the evolution and sustenance of flow-induced phenomena in liquid crystals. This plays a pivotal role in the transport process, effectively providing us the route to guided and/or stereo-selective transport of colloidal particles over large length scales. Such phenomena are in contrast to isotropic fluids. The transport phenomena are characterized using a combination of particle tracking, polarising optical microscopy and fluorescence confocal polarizing microscopy.

13. 3. 2012 - Colloqium at Physics Department

Prof. Mark Dennis: Blue sky research: the polarization of sky light

H. H. Wills Physics Laboratory, University of Bristol, UK

A little-known fact is that the polarization of the natural blue sky of daylight contains polarization singularities (polarization vortices), where the degree of polarization vanishes. They were discovered by observation in the 1800s, explained approximately by Rayleigh scattering, and their detailed prediction was one of the successes of Chandrasekhar's radiative transfer theory in the 1950s. Here, I will discuss their topological properties: as simple examples of polarization singularities, they organize the pattern of polarization in skylight. A simple mathematical ansatz is consistent both with observations and Chandrasekhar's theory, and justified in terms of an analytic approximation of radiative transfer. The ansatz is closely related to polarization patterns in crystal optics, and is related to the polarization of the cosmic microwave background radiation.

5. 12. 2011 - Colloqium at Physics Department

Prof. Tim Sluckin: Magnetic liquid crystal colloids: history, theory and practice

University of Southampton UK and University of Maribor

Liquid crystals align along (or depending on the properties of the molecules involved, sometimes perpendicular to) magnetic and electric fields. The key property of liquid crystals which enables their use in devices is the so-called Frederiks effect. This involves the existence of a threshold field in a cell at which the liquid crystal alignment switches from that dictated by the walls of the cell to that dictated by the applied magnetic or electric field. In devices, this is an electric field, and the critical voltage is of the order of a volt. The equivalent magnetic field is enormous, and magnetic switching in devices is impracticable. In 1970 the future Nobel prize winner Pierre-Gilles de Gennes suggested that the Frederiks threshold magnetic field could be massively reduced if magnetic colloid particles were added to the liquid crystal. Under such circumstances magnetic liquid crystal devices might become practical. In this talk I shall explore this idea, and discuss recent theoretical and experimental work on what have come to be known as "ferronematic" systems.

28. 7. 2011 - Softmatter Seminar

Dr. Teresa Lopez-Leon: Liquid crystal shells: A strategy to generate colloids with directional-binding capabilities

Universite Montpellier II

The fascinating idea of mimicking molecular chemistry with colloids (nano-objects) has motivated the research of scientists in many fields. However, colloids typically lack the ability to bond along specific directions as atoms and molecules do, which limits the complexity of the structures that they can spontaneously form. In a seminal paper, it was suggested that liquid crystal properties could be exploited to tackle this problem. The idea is to coat spherical colloidal particles with a thin layer of nematic liquid crystal and add ligands to the unavoidable defects or bald spots that arise when the nematic order is established on the surface of a sphere. Such ligands would act as linkers between particles and direct their interactions along specific directions, which are determined by the defect positions. Despite the big impact that these nano-structured systems could have in photonics or material science, this topic has been barely explored experimentally, mainly because of the experimental difficulties associated to the fabrication of a nematic shell. We have overcome this problem by taking advantage of a novel microfluidic technique that enable a controlled fabrication of multiple emulsions, i. e., droplets containing smaller droplets inside. Specifically, our nematic shells are double emulsions that consist of droplets of nematic liquid crystals containing a single aqueous droplet inside. When the size of the inner droplet is comparable to the size of the outer one, the aqueous core is coated by a thin nematic shell. With these experimental shells, we have not only corroborated theoretical predictions, but we have been able to systematically control the number and position of the defects formed. For thin shells, we have shown that these defects can be engineered to emulate the linear, trigonal and tetrahedral directionalities of sp, sp2 and sp3 carbon bonds.

13. 7. 2011 - Softmatter Seminar

Dr. Robert King: Fibred Optical Vortex Knots

University of Bristol

Optical fields propagating in three-dimensional free space are complex scalar fields, and typically contain nodal lines (optical vortices) and these can be thought of as defect lines in three dimensions. I will describe how we construct knotted optical vortex lines that are isolated. Using the theory of fibred knots, I will show how we use braided lines to ultimately create experimentally realisable laser mode superpositions with a prescribed knotted vortex. These superpositions are solutions of the 2+1D Schrödinger (paraxial) equation.

24. 5. 2011 - Softmatter Seminar

Dr. Surajit Dhara: Rheological properties of some unconventional and conventional thermotropic liquid crystals

School of Physics, University of Hyderabad, India

The structure-property relationship of conventional and unconventional liquid crystals has always been interesting from both technological and fundamental points of view. It is observed that the physical properties and, in particular electro-optical responses, in unconventional liquid crystals (e.g., bent-core liquid crystals) are very different compared with the calamitic (rod-like) liquid crystals, and the molecular shape has significant effect on phase structure and macroscopic physical properties. In this talk I will present and briefly discuss the viscoelastic properties of some conventional and unconventional liquid crystals and their mixtures.

23. 3. 2011 - Softmatter Seminar

Dr. Jun-ichi Fukuda: Skyrmions and exotic defect structures in a confined chiral liquid crystal

National Institute of Advanced Industrial Science and Technology, Japan and Faculty of Mathematics and Physics, University of Ljubljana, Slovenia

We investigate numerically the structures of a chiral liquid crystal confined in a thin parallel cell whose surfaces impose homeotropic or fixed planar anchoring. We are interested in the cases in which cholesteric blue phases are stable in the bulk, and discuss how the structures of blue phases are changed in the presence of confining surfaces. We find various exotic defect structures completely different from those of bulk cholesteric blue phases. In the case of homeotropic surface anchoring, depending on the cell thicknesses and temperatures, we observe, for example, a parallel array of disclination lines forming double helices, and a hexagonal array of point-like excitations referred to as Skyrmions. When the surfaces adopt fixed planar anchoring and the cell thickness is incommensurate with the intrinsic cholesteric pitch, the frustration due to the incommensurability is relaxed to form a regular array of ring defects.

16. 2. 2011 - Softmatter Seminar

Dr. Richard James: Fringing-Field Induced Defects in Nematic Liquid Crystals

Ghent University, Department of Electronics and Information Systems, St. Pietersnieuwstraat 41, 9000 Gent, Belgium

Fringing fields between pixels of a liquid crystal cell can cause abrupt changes in orientation. Electric field strengths above some threshold can lead to order melting and in turn defects. Accurate modelling of defects induced by fringing fields in a nematic liquid crystal will be presented, calculated by means of the Landau-de Gennes theory and discretised using the finite element method. Defect paths are resolved taking into account flow of the liquid crystal. Voltage thresholds will be shown as reducible through the use of weak anchoring surfaces or optimisation of the electrode dimensions.

19. 1. 2011 - Softmatter Seminar

Regina Jose: Novel Monte Carlo simulation methods to study liquid crystal systems

Faculty of Mathematics and Physics, University of Ljubljana, Slovenia and University of Hyderabad, Hyderabad, India

A liquid crystal film confined to homeotropic substrates tends to condense into a director structure (A) which is uniformly homeotropic, corresponding to the predictions based on minimization of the relevant free energy; and the second structure (B) with a planar director configuration in the middle of the film which manifests itself as a possible metastable state. We employ a novel Monte Carlo (MC) method based on entropic sampling technique as well as canonical method to study such a confined LC system. In order to further investigate such metastable states, we developed a method, (S-sampling) which allows us to sample the very low probable microstates by employing a random walk in the order parameter space.

In the talk, the Wang-Landau (WL) based MC method and further development to S-sampling method will be discussed and compared with the conventional Metropolis algorithm as applied to the afore-mentioned system.

The WL method enables us to investigate the systems with a very high resolution in temperature and hence is used to explore the effect of external field near the paranematic-nematic transition, the results of which agree qualitatively with that of earlier experimental studies.

7. 6. 2011 - Colloqium at Physics Department

Prof. Nicholas L. Abbott: Effects of Confinement on the Ordering of Liquid Crystals: Surprises that Lead to Sensors

University of Wisconsin-Madison, Madison WI, USA

This presentation will address recent experimental investigations of the ordering of liquid crystals within micrometer-sized droplets that are dispersed in water. The first part of the talk will describe the design of an experimental system that allows precise control over the size and interfacial chemistry of liquid crystalline droplets. Experimental observations of size-dependent ordering of the liquid crystals within the droplets will be shown to differ from widely reported scaling arguments for the relative contributions of surface and elastic effects to the free energy of confined liquid crystals. The second part of the talk will address unexpected observations regarding the effects of picogram per milliliter concentrations of bacterial endotoxin on ordering transitions in micrometer-size liquid crystalline droplets. The ordering transitions, which occur at surface concentrations of endotoxin that are less than 10-5 Langmuir, are not due to adsorbate-induced changes in the interfacial energy of the liquid crystal. The sensitivity of the liquid crystal to endotoxin was measured to change by six orders of magnitude with the geometry of the LC (droplet versus slab), providing support for our hypothesis that the interactions of the endotoxin with topological defects in the liquid crystal mediate the response of the droplets. The liquid crystal ordering transitions depend strongly on glycophospholipid structure and provide new designs for highly sensitive sensors for bacterial contamination.

2. 6. 2011

Prof. V.G. Chigrinov: Switchable liquid crystal cells: from field sequential color displays to E-paper

Hong Kong University of Science and Technology, CWB, Hong Kong

The response time of liquid crystal displays is still inferior to the existent competing technologies, such as plasma display panels (PDP) or cathode ray tubes (CRT). The target LC cell response time for a field sequential color (FSC) displays should be less than 1 ms (240 Hz frame frequency) to provide a high resolution, low power consumption and extended color gamut liquid crystal displays (LCD). One of the candidate for fast switching LC materials is a blue phase. However, these materials suffer a narrow temperature range and too high applied voltage and power consumption. Fast switching ferroelectric liquid crystal (FLC) displays (FLCD) is a good candidate for the new generation of field sequential color (FSC) LCD, which proved to be better in response time, than usually used nematic LC.

Optical rewritable technology (ORW) is a method of azo-dye photoalignment with reversible in-plane aligning direction reorientation, i.e. rotation perpendicular to the polarization of an incident light. The photoaligned optically rewritable (ORW) liquid crystal displays (LCD is capable to write, store and rewrite again the information on a glass or flexible carrier. The possible but not limiting applications of the new optically rewritable (ORW) liquid crystal displays (e-paper) based on photoaligning are light printable rewritable paper, labels and plastic card displays, as well as rewritable paper for security applications.

25. 5. 2011 - Colloqium at Physics Department

Prof. Mark Dennis: Knotting Nothing: Tangled Topological Defects in Light

H. H. Wills Physics Laboratory, University of Bristol, UK

Optical fields propagating in three-dimensional free space are complex scalar fields, and typically contain nodal lines (optical vortices) which may be thought of as defect lines in three dimensions.

Random wave fields, representing speckle patterns randomly scattered from rough surfaces, have a tangled skeleton of nodal lines, some of which are closed loops, and others are infinite, open lines. We have found, in computer simulations of random superpositions of plane waves, that these lines have the fractal properties of brownian random walks with characteristic scaling of the probability that pairs of loops are linked together.

Holographically-controlled laser beams provide the opportunity to control the form of optical fields and the nodal lines within them. Using the theory of fibred knots, I will describe the design of superpositions of laser modes (effectively solutions of the 2+1 Schrödinger equation) which contain isolated knots and links. I will conclude by explaining how these mathematical fields have been experimentally realized by the experimental Optics group in Glasgow.

6. 4. 2011 - Colloqium at Physics Department

Prof. Helen Gleeson: Pushing, pulling and twisting liquid crystals with laser tweezers

School of Physics and Astronomy, University of Manchester, UK

Laser tweezers are a beautiful tool to use with liquid crystal systems because the forces they exert (picoNewtons) are of the same order as those associated with deformations in liquid crystals. While laser tweezers have been used to probe defects in liquid crystals and to create beautiful colloidal structures, this talk focuses on two dynamic applications. In the first, laser tweezers are used to measure local viscosity coefficients in liquid crystals. They are a unique tool to do so as they allow us to work in the low Ericksen regime. In the second application, we demonstrate how liquid crystalline droplets can be rotated in laser tweezers, forming all-optical switches. Finally we show a unique system where linearly polarized lasers can induce a continuous rotation of some special liquid crystal droplets.

16. 3. 2011 - Colloqium at Physics Department

Prof. Hideo Takezoe: Liquid Crystal Displays: Principle, current status and prospect

Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Japonska

After introducing the principle of liquid crystal display (LCD) and manufacturing, I will summarize the problems such as viewing angle, contrast ratio, and switching speed, and tell you how they can be solved. I will also tell you the current status of LCDs. Finally I will summarize the prospect of LCD. Some more advanced LCDs such as 3D-LCDs and dual- view LCDs, will be described.

1. 2. 2011 - Colloqium at Physics Department

Prof. Maria Helena Godinho: New Stimuli-Responsive Cellulosic Liquid Crystalline Materials

Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Portugal

Cellulose is an abundant and renewable resource found in most parts of the world. Moreover cellulosic systems can generate liquid crystalline phases. In this work we report on liquid crystalline cellulosic fibers and jets, which mimic the shapes of helical tendrils of climbing plants such as Passiflora edulis and other structures with intrinsic curvature. Liquid crystalline and isotropic cellulosic precursor solutions of curved and straight fibers are examined using nuclear magnetic resonance imaging (MRI) and polarizing optical microscopy (POM) techniques to determine morphological and structural features contributing to fiber curvature. In the case of liquid-crystalline cellulosic fibers, this is due to a core of disclination forming off-axis along the filament. We also highlight the critical dependence of the helical structures on temperature and other external stimulus.

14. 1. 2011 - Colloqium at Physics Department

Dr. George Cordoyiannis: High-resolution calorimetry of phase transitions and critical phenomena in soft materials

Institut Jožef Stefan & EN FIST Center odličnosti Ljubljana, Slovenija

High-resolution calorimetric techniques are extensively used in order to determine the phase transition behaviour of various systems, ranging from binary liquids, to soft and solid materials. An overview of calorimetric techniques, from the popular differential scanning calorimetry to the high-resolution adiabatic scanning and ac calorimetry, will be given. The relative advantages and disadvantages of these techniques will be discussed. The use of high-resolution calorimetry in the study of phase transitions and critical phenomena will be demonstrated. In a first case, experimental results on the Isotropic to Nematic transition of liquid-crystalline elastomers will be presented. The link between the critical behaviour and the giant thermo-mechanical response of these materials will be explained. In a second case, experimental evidence on the weakly first order character of the Nematic to Smectic A phase transition in liquid crystals will be presented. The results will be discussed within the frame of the Halperin-Ma-Lubensky argument and the formulation proposed by Anisimov and co-workers.

11. 11. 2010 - Colloqium at Physics Department

Prof. Mojca Čepič: Kaj je vzrok mnogoplastnih struktur v antiferoelektričnih tekočih kristalih?

Pedagoška fakulteta, Univerza v Ljubljani in Jožef Stefan Institut

Antiferoelektrični tekoči kristali so pred dvema desetletjema vzbudili pozornost zaradi preklapljanja v električnem polju, ki je vključevalo tri stabilna stanja in obetalo zanimive možnosti za uporabo. Hkrati z odkritjem teh lastnosti, pa so odkrili tudi obstoj cele vrste faz, katerih strukturo so raziskovalci poskušali odkriti in potrditi kar dolgih deset let. Čeprav še do danes ni jasno, ali imajo tudi te dodatne faze aplikativni potencial, pa so iz akademskega stališča izredno zanimive in pomembne tudi za gradnjo razumevanja mnogih konceptov v fiziki mehke materije. Antiferoelektrični tekoči kristali tvorijo plasti z nagnjenimi molekulami glede na plastno normalo. Smer nagiba se iz plasti v plast spreminja, način spreminjanja pa je značilnost posameznih faz. V prispevku se bom osredotočila na naslednje:

- Kateri pogoji morajo biti izpolnjeni, da se pojavijo strukture, katerih periode modulacij segajo čez 3, 4 ali v lanskem letu odkrite strukture segajoče čez 6 plasti?

- Ali zahtevajo te kompleksne strukture tudi interakcije med oddaljenimi plastmi?

18. 3. 2010 - Colloqium at Physics Department

Prof. Antonio DeSimone: Mechanics of nematic elastomers: modeling, analysis, and numerical simulation

SISSA-International School for Advanced Studies, Trieste, Italy

Nematic elastomers are systems which combine optical properties of nematic liquid crystals with the mechanical properties of rubbery solids. They display phase transformations, material instabilities, and microstructures. These phenomena are related to the formation of elastic shear bands which are reminiscent of mechanical twinning in shape-memory alloys. The richness of the underlying material symmetries makes the mathematical analysis of this system particularly rewarding. In this talk, we will review the recent progress on the modelling of martensitic-like microstructures in nematic elastomers, which has led to accurate coarse-grained models for the effective mechanical response.

2. 3. 2010 - Colloqium at Physics Department

Prof. Kurt Kremer: The role of topological constraints in soft matter physics

Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany

Soft Matter covers most biological systems and a huge class of everyday products ranging from simple plastics, high tech polymers for electronics to most foods. Its properties are mostly governed by the statistical mechanics of strongly fluctuating huge molecules, such as polymers. For this the fact that polymer chains cannot cross through each other is of central importance to many aspects of soft matter physics being rheological properties of synthetic polymers or so called chromosome territories in biophysics. However, despite its relevance, the theoretical treatment still poses many unsolved questions. In the present talk, after a general introduction into soft matter physics, the role of topological constraints on conformational as well as dynamical properties for a variety of macromolecular systems will be discussed and compared to experiment and simulation results.