Electrohydrodynamic instability in nematics

Various other instances of hydrodynamic and electrohydrodynamic instabilities in nematic and, to a lesser extent, smectic liquid crystals have been investigated. No attempt is made here to review this work. For the present discussion, it is sufficient to note that (a) most of the work has dealt with oriented layers having anisotropic properties, and (b) some interesting instabilities arise in oriented layers which do not occur for isotropic materials. An example of the latter is cellular convection in a fluid layer confined between horizontal plates maintained at different temperatures. With an isotropic fluid, convection can arise only if the lower plate is hotter than the upper plate. Then, fluid near the lower plate is less dense and tends to rise while fluid near the upper plate is denser and tends to sink. With an oriented layer, however, convection can arise even when the upper plate is hotter if the anisotropy of thermal conduction properties is of a particular type (8). 

L. Kramer and W. Pesch, Electrohydrodynamic Instabilities in Nematic Liquid Crystals. In Pattern Formation in Liquid Crystals, editors A. Buka and L. Kramer, pages 221-255, Springer, 1996. 

A.N. Trufanov, M.I. Barnik and L.M. Blinov, A Novel Type of the Electrohydrodynamic Instability in Nematic Liquid Crystals, m Advances in Liquid Crystal Research and Application, editor L. Bata, pages 549-560, Akademiai Kiado - Pergamon Press (1980). 

N.V. Madhusudana, V.A. Raghunathan and K.R. Sumathy, Plexoelectric origin of oblique-roll electrohydrodynamic instability in nematics, Pramana-J. Phys. 28(3), L311-L316, (1987). 

E.I. Rjumtsev and S.G. Polushin, Electrohydrodynamic instabilities in nematic liquid crystals with large positive dielectric anisotropy, Liq. Cryst. 13(5), 623-628, (1993). doi 10.1080/02678299308026335... 

FIGURE 5.5. Electrohydrodynamic instability in nematic liquid crystals (a) the onset of the instabihty (b) the vortex motion of a Hquid crystal and (c) the picture of black-and-white stripes in the screen plane. 

Electrohydrodynamic Instability in Nematics with Oblique Director Orientation at the Boundaries... 

FIGURE 5.10. Electrohydrodynamic instabilities in nematic liquid crystal with homogeneous initial orientation in the high-frequency regime (a > lJc)- (a) Domains when the voltage is slightly above the threshold voltage and (b) the Chevron pattern. 

Electrohydrodynamic instabilities in nematics could be classified according to the dependence of the threshold voltage (or field) on the physical parameters of the liquid crystal, cell geometry, field firequency, etc. Arising domain patterns also differ by the period of the structure and its orientation with respect to the initial director. We hope that this classification proves to be useful, both for finding similar instability phenomena in other liquid crystals (cholesteric, smectic, polymer liquid crystals, etc.) and for practical purposes in avoiding parasitic scattering and hysteresis effects which are undesirable in many applications. 

A.N. Trufanov, Investigation of mechanisms of electrohydrodynamic instabilities in nematic liquid crystals. Doctor Thesis, Institute of Crystallography, Moscow, 1981. 

L. Kramer, W. Pesch Electrohydrodynamic instabilities in nematic liquid crystals, in A. Buka, L. Kramer (eds) Pattern formation in liquid crystals. Springer, New York, (1995)... 

D. Meyerhofer and A. Sussman, Electrohydrodynamic Instabilities in Nematic Liquid Crystals in Low-Frequency Fields, Appl. Rhys. Lett., 20, p. 337 (1972). 

Orsay Liquid Crystal Group, AC and DC Regimes of the Electrohydrodynamic Instabilities in Nematic Liquid Crystals, Mol. Cryst. andUq. Cryst, 12, p. 251 (1971). 

The second group of effects are phenomena of electrohydrodynamic instability (EHD) caused by the movement of a liquid in an external electric field. In liquid crystals, this instability only arises for sufficiently high impurity conductivity of the substance. There are two mechanisms of the onset of electrohydrodynamic instability in nematic liquid crystals. One of them, isotropic, is not specific for liquid crystals and can be observed for any values of Ae and Aa. In this case, the electrohydrodynamic process consists of movement of the liquid in an electrical force gradient which arises due to the inhomogeneous charge and field distribution in the bulk of the sample. 

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