Electrohydrodynamic effects

The slopes of the different curves correspond to the fuU electrohydrodynamic effect, ( ) + ( ) pj, where the first term expresses the hydrodynamic effect, and the second is the consequence of the distortion of the electrical double layer that surrounds the particles. To determine this second term and, more exactly, the primary electroviscous coefficient, pi. 

All electrooptical effects known to the present time for polymeric liquid crystals may be divided into two groups. First of all there are so called orientational effects, which are due solely to the effect of the electric field (field effect) on LC polymers, but are not a result of a current flowing. The second group of electrooptical effects is attributed to the phenomena ascribed to the anisotropy of electrical conductivity (Act) of liquid crystals. These are called electrohydrodynamic effects. 

At the boundary between the phases, one of which is an electrolyte solution, a region of charged solution is formed - an electric double layer whose presence causes peculiar kinds of electrohydrodynamic effects that manifest themselves in the motion of particles in the electrolyte solution and are called electrokinetic phenomena. All electrokinetic phenomena have common mechanism and arise from the relative motion of phases [3, 23]. 

Induced-charge and second-kind electrokinetic phenomena arise due to electrohydrodynamic effects in the electric double layer, but the term nonlinear electrokinetic phenomena is also sometimes used more broadly to include any fluid or particle motion, which depends nonlinearly on an applied electric field, fit the classical effect of dielectrophoresis mentioned above, electrostatic stresses on a polarized dielectric particle in a dielectric liquid cause dielectro-phoretic motion of particles and cells along the gradient of the field intensity (oc VE ). In electrothermal effects, an electric field induces bulk temperature gradients by Joule heating, which in turn cause gradients in the permittivity and conductivity that couple to the field to drive nonlinear flows, e.g., via Maxwell stresses oc E Ve. In cases of flexible solids and emulsions, there can also be nonlinear electromechanical effects coupling the... 

The only description of electric field orientation comes out of work by Ober and co-workers (37,113). They used ac electric fields, which allowed them to control orientation either parallel or perpendicular to the field direction. This is caused by an electrohydrodynamic effect, by which there exists a critical field frequency, below which the molecule orients parallel to the electric field and above which the molecule orients perpendicular to the electric field. Ober and co-workers were able to show control over the direction of orientation in a cyanate ester LOT by changing the frequency of the applied field during the cure (113). In contrast, an epoxy LOT flipped from a parallel to a perpendicular orientation during isothermal cure at a given applied frequency (37). This was attributed to a change in the critical frequency due to the increase in viscosity during cure. 

Induced-charge and second-kind electrokinetic phenomena arise due to electrohydrodynamic effects in the electric double layer, but the term nonlinear electrokinetic phenomena is also sometimes used more broadly to include any fluid or particle motion, which depends nonlinearly on an applied electric field. In the classical effect of... 

Electrohydrodynamic effects in liquid crystals are connected with an electrical current and a mechanical fluid flow. Consequently, the anisotropic electric conductivity and the various viscosity parameters play an important role. 

Several electrohydrodynamic effects are known. The most important effect is dynamic scattering as described in 1968 by the RCA liquid crystal group [60]. Already in 1918 Bjornstahl [61] reported measurements on the extinction of light transmitted through a nematic liquid crystal under the influence of an electric field. With a 6.2 mm thick layer of p-azoxyphenetole, he observed a threshold field of 100 V/cm, above which the extinction increased sharply. 

A Sussmann found that the useful lifetime of a d.c. driven cell is inversely proportional to the current density in the cell [83]. On the other hand the electrical conductivity of the liquid crystal determines the contrast of a display based on an electrohydrodynamic effect such as dynamic scattering. The conductivity can therefore not be decreased at will in favor of longer display lifetime. For optimum contrast the conductivity is about 10 (i2 cm)" to 10 (f2 cm)" ... 

An advantage of magnetic fields over electric fields for controlling alignment is that complications due to electrical conduction or electrohydrodynamic effects are not present. Competition between aligning fields has been used to obtain direct measurements of susceptibility anisotropies. The basis of the method can be understood from Eq. (5). A similar equation can be written for the free energy density of a liquid crystal in an electric field, such that ... 

Electrohydrodynamic effects in nematics II", Comments Solid State Phvs.. 2, 148 (1971). 

F. Rondelez, H. Arnould and C. J. Gerritsma, Electrohydrodynamic Effects in Cholesteric Liquid Crystals Under AC Electric Fields, Rhys. Rev. Lett., 28, p. 735 (1972). 

P. DeGennes, Electrohydrodynamic Effects in Nematic Liquid Crystals I. DC Effects, in Comments on Solid State Physics, Vol. 3, p. 35 (1970) and P. A. Penz, Electrohydrodynamic Solutions for Nematic Liquid Crystals with Positive Dielectric Anisotropy, Mol. Cryst, Vol. 23, p. 1 (1973). 

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