Electric-field-mediated instability modes and Fréedericksz transition of thin nematic films

Abstract

Instabilities at the deformable free surface of a thin nematic liquid crystal film can develop interesting patterns when exposed to an external electrostatic field. A general linear stability analysis is performed involving the Ericksen–Leslie governing equations for the dynamics of the nematic film coupled with the anisotropic Maxwell stresses for the electric field to uncover the salient features of these instabilities. The study reveals the coexistence of twin instability modes: (i) long-wave interfacial mode – stimulated when the sole destabilizing influence of the electric field overcomes the Frank bulk elasticity and surface tension force, and (ii) finite-wavenumber mode – engendered by the combined destabilizing influence originating from the anisotropic electric field and Ericksen stress, for the films with positive dielectric anisotropy and weaker Frank bulk elasticity. The results reported here are in contrast with the same obtained from the more frequently employed long-wave approach. The air-to-liquid-crystal filling ratio between the electrodes as well as thermodynamic parameters such as the dielectric anisotropy, Frank elasticity, and director orientations across the film and boundaries are found to play crucial roles in the selection of modes, whereas kinetic parameters such as Leslie viscosity coefficients influence only the time scale of instability. Importantly, at higher field intensities a symmetry-breaking Fréedericksz-type transition of director orientations is found to happen, which also causes the transition of the dominant mode of instability from the long-wave to the finite-wavenumber mode for films with relatively lower values of Frank bulk elasticity and positive dielectric anisotropy.

Phase transition and dewetting of a 5CB liquid crystal thin film on a topographically patterned substrate

“…It should be mentioned that the Laplace pressure at the nematic phase is different than the isotropic phase due to different molecular orientation. 53 Further, during cooling (I-N), it is observed that the texture of the lm does not change but shows new texture with few disclination points. We believe that aer achieving minimum energy conguration, the orientation of bulk molecules do not change.…”

Translating 2D Director Profile to 3D Topography in a Liquid Crystal Polymer

“…Electricallystimulated instabilities have also been observed in low-molecular weight liquid crystals. [29][30][31] In that case the liquid surface is destabilized forming temporary undulations. To the best of our knowledge, it is the first time that such phenomenon is described in LCPs with the advantage that permanent 3D structures are being formed.…”

Numerical analysis of electrohydrodynamic instability in dielectric-liquid–gas flows subjected to unipolar injection

“…For example, it has been proved that the electric field is able to produce many different flow. patterns in the absence of shear flow between layers [15][16][17]. Some typical interface instabilities between two fluid layers like the Rayleigh-Taylor instability can also be controlled by electric field [18][19][20].…”