Electrohydrodynamic Convection in Nematics: Hybrid and Tilted Alignment

Abstract

We calculate the threshold of electrohydrodynamic convection (EHC) for nematic liquid crystals with negative dielectric anisotropy, where one allows for different director orientations at the upper and lower confining plates (hybrid cells). Besides the planar versus homeotropic case we present for the first time an analysis of continuous tilt at one plate. The predistorted ground state strongly influences the EHC character and the roll orientation. One observes a generic nonstationary behaviour and in particular the interesting scenario of drifting oblique rolls

Flashing flexodomains and electroconvection rolls in a nematic liquid crystal

“…lack of a sharp threshold) in the case of splay Freedericksz-transition. The effect of a tilted alignment on the EC characteristics has theoretically been studied only for high frequencies [29]; the pretilt modified U c , but did not affect the sharpness of the threshold, which is in agreement with our observations (Fig. 5) at high f .…”

Drifting Abnormal Rolls in Electroconvection of Hybrid Aligned Nematic

“…In our calculations we used the standard set of nematodynamic equations [4,8]. Below onset of convection one has the director configuration distorted across the layer due to the opposite boundary conditions and no material flow is excited.…”

“…Below onset of convection one has the director configuration distorted across the layer due to the opposite boundary conditions and no material flow is excited. The linear stability analysis of this state is based on a Galerkin method (see [8] for details). Material parameters of MBBA at 25 • C [8] were used with the only exceptions the values of conductivities σ || /σ ⊥ = 1.71 and σ ⊥ = 0.55 · 10 −8 (Ω·m) −1 which were fitted from the experimental data on the threshold voltage U c at low frequency of a.c. voltage and the cut-off frequency f c , respectively.…”

“…Below onset of convection one has the director configuration distorted across the layer due to the opposite boundary conditions and no material flow is excited. The linear stability analysis of this state is based on a Galerkin method (see [8] In order to reveal the direction of the mean drift velocity, small tracer particles (2−4 µm in diameter) were immersed in the nematic. The observations show that the particles move on closed trajectories within EC rolls in the plane perpendicular to the roll axis and also drift together with rolls.…”

“…In comparison with the case of uniform director alignment (planar or homeotropic) only few works were devoted to EC in nematics with more complex director distribution, e.g., in hybrid aligned nematics (HAN) [7,8]. Here below onset the director interpolates from the planar alignment at one confining plate to homeotropic orientation at the other one.…”