Showing papers by "Samo Kralj published in 2017"

Impact of ferroelectric and superparaelectric nanoparticles on phase transitions and dynamics in nematic liquid crystals

Abstract: Results of broadband dielectric spectroscopy (BDS) studies of pure liquid crystalline (4-pentyloxy-4-biphenylcarbonitryle) 5OCB and its nanocolloids with BaTiO_{3} nanoparticles (NPs) under varying pressure and temperature are presented. The notable impact of NPs on phase transitions and dynamics was found. Particularly strong impact on pretransitional behavior was observed for relatively low concentrations of NPs, which can be related to the NPs-induced disorder. There are also notable differences between pressure and temperature paths of studies for nanocomposites, absent for the pure LC compound. For instance, tests focused on the translational orientational decoupling via the fractional Debye-Stokes-Einstein relation yielded S=0.71 and S=0.3 for the temperature and pressure paths, respectively: S=1 is for the complete coupling. The possible theoretical frame of observed phenomena is also proposed.

On the role of external force of actin filaments in the formation of tubular protrusions of closed membrane shapes with anisotropic membrane components.

Abstract: Biological membranes are composed of different components and there is no a priori reason to assume that all components are isotropic. It was previously shown that the anisotropic properties of membrane components may explain the stability of membrane tubular protrusions even without the application of external force. Our theoretical study focuses on the role of anisotropic membrane components in the stability of membrane tubular structures generated or stabilized by actin filaments. We show that the growth of the actin cytoskeleton inside the vesicle can induce the partial lateral segregation of different membrane components. The entropy of mixing of membrane components hinders the total lateral segregation of the anisotropic and isotropic membrane components. Self-assembled aggregates formed by anisotropic membrane components facilitate the growth of long membrane tubular protrusions. Protrusive force generated by actin filaments favors strong segregation of membrane components by diminishing the opposing effect of mixing entropy.

Liquid crystals with nano and microparticles, edited by Jan P. F. Lagerwall and Giusy Scalia, Singapore, World Scientific, 2017, 2 volume set, 920 pp., £328 ($435, 370 Euro) (hardback), ISBN: 978-981-4619-25-7

Abstract: The book addresses complex configurations and superstructures emerging from interactions between liquid crystal (LC) phases and immersed nano- or micro-particles. Its content merges two classical s...

Decomposition of strongly charged topological defects.

Abstract: We study decomposition of geometrically enforced nematic topological defects bearing relatively large defect strengths m in effectively two-dimensional planar systems. Theoretically, defect cores are analyzed within the mesoscopic Landau-de Gennes approach in terms of the tensor nematic order parameter. We demonstrate a robust tendency of defect decomposition into elementary units where two qualitatively different scenarios imposing total defect strengths on a nematic region are employed. Some theoretical predictions are verified experimentally, where arrays of defects bearing charges m=±1 and even m=±2 are enforced within a plane-parallel nematic cell using an atomic force microscopy scribing method.

Decomposition vs. escape of topological defects in a nematic liquid crystal.

Abstract: Nematic cells patterned with square arrays of strength m = ±1 topological defects were examined as a function of cell thickness (3 < h < 7.5 μm), temperature, and applied voltage. Thicker cells tend to exhibit an escape or partial escape of the nematic director as a means of mitigating the elastic energy cost near the defect cores, whereas thinner cells tend to favor splitting of the integer defects into pairs of half-integer strength defects. On heating the sample into the isotropic phase and cooling back into the nematic, some apparently split defects can reappear as unsplit integer defects, or vice versa. This is consistent with the system's symmetry, which requires a first order transition between the two relaxation mechanisms.

Twist-grain boundary phase induced by Au nanoparticles in a chiral liquid crystal host

Abstract: We report on the stabilisation of the liquid-crystalline, twist-grain boundary A (TGBA) phase in mixtures of a chiral liquid crystal and surface-functionalised spherical Au nanoparticles (NPs) of 1...

Curvature-Controlled Topological Defects

Abstract: Effectively, two-dimensional (2D) closed films exhibiting in-plane orientational ordering (ordered shells) might be instrumental for the realization of scaled crystals In them, ordered shells are expected to play the role of atoms Furthermore, topological defects (TDs) within them would determine their valence Namely, bonding among shells within an isotropic liquid matrix could be established via appropriate nano-binders (ie, linkers) which tend to be attached to the cores of TDs exploiting the defect core replacement mechanism Consequently, by varying configurations of TDs one could nucleate growth of scaled crystals displaying different symmetries For this purpose, it is of interest to develop a simple and robust mechanism via which one could control the position and number of TDs in such atoms In this paper, we use a minimal mesoscopic model, where variational parameters are the 2D curvature tensor and the 2D orientational tensor order parameter We demonstrate numerically the efficiency of the effective topological defect cancellation mechanism to predict positional assembling of TDs in ordered films characterized by spatially nonhomogeneous Gaussian curvature Furthermore, we show how one could efficiently switch among qualitatively different structures by using a relative volume v of ordered shells, which represents a relatively simple naturally accessible control parameter

Impact of curvature on topological defects

Abstract: We analyze the impact of extrinsic and intrinsic curvature on positions of topological defects (TDs) in two-dimensional (2D) nematic films. We demonstrate that both these curvature contributions are commonly present and are expected to be weighted by comparable elastic constants. A simple Landau-de Gennes approach in terms of tensor nematic order parameter is used to numerically demonstrate impact of the curvatures on position of TDs on 2D ellipsoidal nematic shells. In particular, in oblate ellipsoids the extrinsic and intrinsic elastic terms enforce conflicting tendencies to positions of TDs.

Impact of diffusion limited aggregates of impurities on nematic ordering

Abstract: We study the impact of random bond-type disorder on two-dimensional (2D) orientational ordering of nematic liquid crystal (LC) configurations. The lattice Lebwohl–Lasher pseudospin model is used to model orientational ordering perturbed by frozen-in rod-like impurities of concentration p exhibiting the isotropic orientational probability distribution. The impurities are either (i) randomly spatially distributed or (ii) form diffusion limited aggregation (DLA)-type patterns characterized by the fractal dimensions d f , where we consider cases d f ∼ 1.7 and d f ∼ 1.9 . The degree of orientational ordering is quantified in terms of the orientational pair correlation function G ( r ) . Simulations reveal that the DLA pattern imposed disorder has a significantly weaker impact for a given concentration of impurities. Furthermore, if samples are quenched from the isotropic LC phase, then the fractal dimension is relatively strongly imprinted on quantitative characteristics of G ( r ) .

Dynamic and static light scattering at phase transitions in liquid crystal confined into porous polymer film

Abstract: Autocorrelation function g2() and intensity (I) of the depolarized scattered light were obtained at the study of a dynamic light scattering from the composite liquid crystal (LC) material, consisting of porous polyethylenterephthalate (PET) film filled with oсtylcyanobiphenyl (8CB). The simultaneous analysis of these parameters made possible to extract the temperatures TIN and TNA of the isotropic liquid – nematic (N–I) and nematic – smectic A (N-SmA) phase transitions in LC samples confined into cylindrical pores of diameters in the range of 0.02 ... 2 μm. The dependences of TIN and TNA on the pore diameter were obtained and compared with the theoretical predictions. It was shown that simultaneous investigations of dynamic and static light scattering can be considered as a perspective approach for a study of phase transitions in LCs at strong confinement, which is typical for composite LC materials.