We introduce a new mesoscopic model for nematic liquid crystals (LCs). We extend the particle-based stochastic rotation dynamics method, which reproduces the Navier-Stokes equation, to anisotropic fluids by including a simplified Ericksen-Leslie formulation of nematodynamics. We verify the applicability of this hybrid model by studying the equilibrium isotropic-nematic phase transition and nonequilibrium problems, such as the dynamics of topological defects and the rheology of sheared LCs. Our simulation results show that this hybrid model captures many essential aspects of LC physics at the mesoscopic scale, while preserving microscopic thermal fluctuations.

Stochastic Rotation Dynamics (SRD) is a valuable numerical tool extensively used in many domains of hydrodynamics simulations including colloidal suspensions. We investigate the dynamics of two colloidal particles in the regime of low Reynolds number by means of SRD in 3D. In contrast to well-known analytical and experimental results, no long-range interaction between the suspended particles could be found, independent of the size of the particles and the Mach and Péclet numbers. We attribute this behavior to the compressible nature and low sound velocity in the SRD solvent. The inability of representing long-range interactions poses an important limitation to the applicability of SRD to certain physical systems. We provide an estimation of typical length scales for which SRD can be applied. Published by AIP Publishing. https://doi.

A large optical nonlinearity has been observed for the photo-responsive liquid crystals under the condition that the nematic phase is close to the isotropic condition. The direct observation of the photo-response of a liquid crystal by the time-resolved transient grating phase imaging technique revealed that the optical nonlinearity was caused by the transiently generated phase formed inside the photo-induced isotropic region. A shock-like flow was observed for the formation of the transiently generated phase. Based on the theoretical calculation, we propose that a flow generated at the disordered/ordered interface induced the reorientation of the liquid crystal molecules, thereby generating a larger polarization and ultimately causing the optical nonlinearity.

We consider electroconvection as a response of nematic liquid crystals to an external electric AC field, in the absence of free charge carriers. Previous experimental and theoretical results emphasized charge carriers as a necessary precondition of electroconvection because free-charges in the fluid can respond to an external electric field. Therefore, ionized molecules are considered as responsible for the driving of electroconvective flows. In experiments, finite conductivity is achieved by adding charge-carrying dye molecules or in non-dyed liquid crystals by impurities of the samples. The phenomenon of electroconvection is explained by the Carr-Helfrich theory, supported by numerical simulations. In the present paper, we show that electroconvection may occur also in pure nematic liquid crystals. By means of particle-based numerical simulations we found that bound charges emerge by alignment of polarized liquid crystal molecules in response to the external electric field. In our simulations we could reproduce the characteristic features of electroconvection, such as director-flow patterns, the phase-transition in the voltage-frequency diagram, and dislocation climb/glide motion, which are well known from experiments and hydrodynamic simulations under the assumption of free charge carriers.

It is shown that single-peaked collisionless current sheets in a Harris-type equilibrium spontaneously bifurcate as a result of chaotic scattering of electrons at fluctuating magnetic fields near the center of the sheet, as demonstrated by a 2D kinetic particle-in-cell simulation. For this effect to be simulated explicit particle advancing is necessary, since the details of the electron motion have to be resolved. Unlike previous investigations of triggering bifurcated current sheet (BCS) where initial perturbations or external pressure was applied the bifurcation is spontaneous if thermal noise is taken into account. A spontaneous current sheet bifurcation develops quicker than a tearing mode or other plasma instabilities. It is shown that in the course of the current sheet bifurcation the Helmholtz free energy decreases while the entropy increases, i.e. the new, bifurcated current sheet is in a more propable state than the single-peaked one.