Instability electrohydrodynamic

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). 

One potential application of the work on oriented nematic phases of rodlike molecules is to solutions containing cylindrical micelles. Orientation could be achieved by a shear field or perhaps by an electric field. Gotz and Heckman (9) confirmed the existence of anisotropic electrical conductivity for a concentrated surfactant solution in a shear field. They used their results to show that the solution contained cylindrical rather than platelike micelles. Of course, the magnitude of the electrical conductivity in an aqueous micellar solution should be quite different from that in the nematic phase of an organic material. So the conditions for and types of electrohydrodynamic instabilities could be different as well. 

When the repnlsive forces dne to the snrface charges exceed the forces due to surface tension, the droplets explode as a result of field-induced Rayleigh or electrohydrodynamic instabilities. Mnltiple smaller droplets are generated. 

It has been Icnown for years that a decrease in the phase transition temperature of azobenzene containing LC is induced by trans -cis photoisomerization (23). Smectic LC of 4-alkyl-4 -cyano-biphenyl is subjected to phase transition by photoisomerization of azobenzene leading to a reversible change in the threshold voltage for electrohydrodynamic instability (24). None of them described the concept of image amplification. 

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). 

DC and very low frequency AC voltages produce electrohydrodynamic instabilities in the isotropic phase also (T the threshold being... 

We shall now outline the theory of electrohydrodynamic instabilities proposed by Helfrich and extended by Dubois-Violette, de Gennes and Parodi and Smith et We consider a nematic film of thickness d lying in the xy plane and subjected to an electric field along z. Let the initial unperturbed orientation of the director be along x, and let there also be a stabilizing magnetic field along the same direction. We consider a bend... 

Fig. 3.13.6. The formation of oblique convective rolls in electrohydrodynamic instability. Below the point M, which may be called a triple point , the transition takes place directly to oblique rolls (/= 10 Hz). Beyond M, normal rolls are obtained first, followed by undulatory rolls which then change continuously into oblique rolls (/ = 60 Hz). (Ribotta et...

Thomas Russell is Silvio O. Conte Distinguished Professor, Polymer Science and Engineering Department Director, Energy Frontier Research Center (EFRC), Polymer-Based Materials for Harvesting Solar Energy. His research interests are polymer-based nanoscopic structures, polymer-based nanoparticle assemblies, electrohydrodynamic instabilities in thin polymer films, surface and interfacial properties of polymers, polymer morphology kinetics of phase transitions, and supercritical fluid/polymer interactions. 

This chapter is concerned with experimental measurements of flexo-electric coefficients. After a brief introduction to flexoelectricity in nematic liquid crystaJs, some applications exploiting the flexoelectric effect and the influence of this effect on electrohydrodynamic instabilities are pointed out. Flexoelectricity axises in samples with a splay-bend distortion in the director field and as such its measurement is not as direct as for dielectric constants. The theoretical background needed to analyse electro-optic experiments and extract the flexocoefficients is outlined in Section 2.2. Various experimental techniques that have been developed are described in Section 2.3. These involve cells in which the alignment of the nematic director is homeotropic, or planar or hybrid. In the first case, the interdigitated electrode technique is particularly noteworthy, as it has been used to establish several features of flexoelectricity (1) the effect can arise purely from the quadrupolar nature of the medium, and (2) the dipolar contribution relaxes at a relatively low frequency. 

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). 

L.M. Blinov, M.I. Barnik, V.T. Lazareva and A.N. Trufanov, Electrohydrodynamic instabilities in the hquid crystalline phases with smectic ordering,... 

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... 

Oddy et al. [7] presented a micromixer in which AC electrical fields were used to induce the elec-trokinetic instability (EKI) phenomenon in microscale channels at very low Re5molds numbers, as shown in Fig. 3. EKI is similar to electrohydrodynamic instability, which can be... 

The early work on modeling electrohydrodynamic instabilities performed a linear stability analysis of the electric and fluid interfacial boundary conditions using the transfer relations developed by Melcher [1]. Melcher developed... 

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