Slobodan Žumer

Three-dimensional colloidal crystals in liquid crystalline blue phases

Applications for photonic crystals and metamaterials put stringent requirements on the characteristics of advanced optical materials, demanding tunability, high Q factors, applicability in visible range, and large-scale self-assembly. Exploiting the interplay between structural and optical properties, colloidal lattices embedded in liquid crystals (LCs) are promising candidates for such materials. Recently, stable two-dimensional colloidal configurations were demonstrated in nematic LCs. However, the question as to whether stable 3D colloidal structures can exist in an LC had remained unanswered. We show, by means of computer modeling, that colloidal particles can self-assemble into stable, 3D, periodic structures in blue phase LCs. The assembly is based on blue phases providing a 3D template of trapping sites for colloidal particles. The particle configuration is determined by the orientational order of the LC molecules: Specifically, face-centered cubic colloidal crystals form in type-I blue phases, whereas body-centered crystals form in type-II blue phases. For typical particle diameters (approximately 100 nm) the effective binding energy can reach up to a few 100 kBT, implying robustness against mechanical stress and temperature fluctuations. Moreover, the colloidal particles substantially increase the thermal stability range of the blue phases, for a factor of two and more. The LC-supported colloidal structure is one or two orders of magnitude stronger bound than, e.g., water-based colloidal crystals.

High‐Temperature Phase Transition in KH2PO4

The high‐temperature phase transition in KH2PO4 has been investigated by differential thermal analysis, thermogravimetric analysis, proton magnetic resonance and relaxation, infrared spectroscopy, and x‐ray and quasielastic cold‐neutron scattering. The transition is not connected with a static breaking up of the hydrogen‐bond network but rather with the onset of disordered hindered rotation of the H2PO4 groups around all three axes. It seems that rotation of H2PO4 groups is as well rate determining for the proton conductivity of this crystal.

Assembly and control of 3D nematic dipolar colloidal crystals.

Topology has long been considered as an abstract mathematical discipline with little connection to material science. Here we demonstrate that control over spatial and temporal positioning of topological defects allows for the design and assembly of three-dimensional nematic colloidal crystals, giving some unexpected material properties, such as giant electrostriction and collective electro-rotation. Using laser tweezers, we have assembled three-dimensional colloidal crystals made up of 4 μm microspheres in a bulk nematic liquid crystal, implementing a step-by-step protocol, dictated by the orientation of point defects. The three-dimensional colloidal crystals have tetragonal symmetry with antiparallel topological dipoles and exhibit giant electrostriction, shrinking by 25-30% at 0.37 V μm(-1). An external electric field induces a reversible and controllable electro-rotation of the crystal as a whole, with the angle of rotation being ~30° at 0.14 V μm(-1), when using liquid crystal with negative dielectric anisotropy. This demonstrates a new class of electrically highly responsive soft materials.

Landau–de Gennes modelling of nematic liquid crystal colloids

Phenomenological Landau–de Gennes modelling based on the free energy of nematic liquid crystal colloids is reviewed. Nematic phase, gradient of order, and surface anchoring contributions to the total free energy are used. The numerical finite difference relaxation technique is explained as an efficient tool for the minimisation of the free energy. Effects of the mesh and mesh allocation are discussed. Various conceptually different colloidal structures are calculated to show the universality of the model. Single particles, dipolar–quadrupolar dimers, entangled dimers, dimers bound by escaped hyperbolic rings, and hierarchically patterned Saturn-ring colloidal superstructures are presented.

Mutually tangled colloidal knots and induced defect loops in nematic fields

Colloidal dispersions in liquid crystals can serve as a soft-matter toolkit for the self-assembly of composite materials with pre-engineered properties and structures that are highly dependent on particle-induced topological defects. Here, we demonstrate that bulk and surface defects in nematic fluids can be patterned by tuning the topology of colloidal particles dispersed in them. In particular, by taking advantage of two-photon photopolymerization techniques to make knot-shaped microparticles, we show that the interplay of the topologies of the knotted particles, the nematic field and the induced defects leads to knotted, linked and other topologically non-trivial field configurations. These structures match theoretical predictions made on the basis of the minimization of the elastic free energy and satisfy topological constraints. Our approach may find uses in self-assembled topological superstructures of knotted particles linked by nematic fields, in topological scaffolds supporting the decoration of defect networks with nanoparticles, and in modelling other physical systems exhibiting topologically analogous phenomena.

Quasi-two-dimensional Skyrmion lattices in a chiral nematic liquid crystal

Skyrmions are particle-like topological entities in a continuous field that have an important role in various condensed matter systems, including two-dimensional electron gases exhibiting the quantum Hall effect, chiral ferromagnets and Bose-Einstein condensates. Here we show theoretically, with the aid of numerical methods, that a highly chiral nematic liquid crystal can accommodate a quasi-two-dimensional Skyrmion lattice as a thermodynamically stable state, when it is confined to a thin film between two parallel surfaces imposing normal alignment. A chiral nematic liquid crystal film can thus serve as a model Skyrmion system, allowing direct investigation of their structural properties by a variety of optical techniques at room temperatures that are less demanding than Skyrmion systems discussed previously.

Liquid crystal elastomer-nanoparticle systems for actuation

Liquid crystal elastomers (LCE) are currently of great interest due to conjoining of mesogenic ordering and rubber elasticity, exhibited in their large spontaneous thermally stimulated changes in shape. It has been shown that nanoparticles (nanotubes, photo-isomerisable dyes, magnetic nanoparticles…) can be incorporated into these LCE networks to create a more sensitive network to external stimuli (i.e. strain or stress, optical, electrical, electro-thermal, magnetic…). Here, we briefly summarise the current state of LCE–nanoparticle systems and explain in detail one system utilising carbon nanoparticles integrated at surfaces that may be used for electro-thermal heating of LCE systems.

Enhanced dynamic response of the in-plane switching liquid crystal display mode through polymer stabilization

A significant improvement in the dynamic response time of the in-plane switching nematic liquid crystal mode, useful in flat-panel display applications, is achieved through polymer stabilization. This improvement is achieved by introducing a low-density, stabilizing polymer network that causes the nematic director to favor the zero-field orientation at the expense of transmission and slightly higher drive voltages. We present a simple model that treats the polymer network as an effective field in the general framework of elastic continuum theory.

Geometrical frustration of chiral ordering in cholesteric droplets

Frustration of chiral ordering is explored in cholesteric liquid crystal droplets with planar degenerate anchoring using numerical modeling. Droplets of variable pitches are studied, demonstrating the role of a gradually increasing cholesteric pitch and the corresponding equilibrium structures. All previously known structures are identified but with notable differences. The structures presented with director fields are complemented with a detailed description of the defect regions. The characteristic half-diameter +2 disclination from previous studies is found to be in fact a double-helix of two λ+1 disclination lines, whereas the full-diameter +1 disclination is composed of an alternating series of τ−1/2 and λ+1/2 disclination rings. Finally, two new meta-stable cholesteric structures -Lyre and Yeti- are found, which are characterised by complex compositions of cholesteric disclinations.

Defects in liquid crystals : computer simulations, theory, and experiments

Preface. 1. Classification of defects in liquid crystals H.-R. Trebin. 2. Alignment tensor versus director description in nematic liquid crystals A.M. Sonnet, S. Hess. 3. Liquid crystal colloidal dispersions H. Stark, et al. 4. Computer simulations and defects in confined liquid crystal lattice models C. Chiccoli, et al. 5. Molecular simulations and theory of planar interfaces and defects in nematic liquid crystals M.P. Allen. 6. Topological defect behavior in a quenched nematic liquid crystal R.A. Pelcovits, et al. 7. Restoring forces on nematic disclinations R. Rosso, E.G. Virga. 8. Challenges in the dynamics of point defects A.M. Sonnet, E.G. Virga. 9. Numerical simulation of elastic anisotropy in nematic liquid crystalline polymers H. Tu, et al. 10. Computer Simulations and Fluorescence Confocal Polarizing Microscopy of Structures in Cholesteric Liquid Crystals S.V. Shiyanovskii, et al. 11. Defects and Undulation in Layered Liquid Crystals T. Ishikawa, O.D. Lavrentovich. 12. Liquid crystals under shear: role of defects M. Kleman, C. Meyer. 13. Numerical simulation of defects in quasicrystals H.-R. Trebin. Index.