Electric field induced deformations

Electric field-induced deformation of polyelectrolyte gels has attracted much attention because of the property of smartness. If the size and shape of gels can be controlled as we hope, this may open a new door for gel technology. In this Section, studies on electric field-induced deformation of gels will first be surveyed. 

The electric field-induced deformation of polyelectrolyte gels was first reported by Hamlen et al. in 1965 [4], They observed that an ionic PVA gel fiber, which was placed touching the anode in a 1% NaCl solution, shrank at the anode side as a result of an applied dc EMF of 5 V. When the polarity of the applied voltage... 

H. Gruler and G. Meier. Electric Field-Induced Deformations in Oriented Liquid Crystals of the Nematic Type, Mol. Cryst Liq. Cryst, 16, p. 299 (1972) W. H. deJeu, C. J. (Serritsma, and T. W. Lathouwers, Instabilities in Electric Fields of Nematic Liquid Crystals with Positive Dielectric Anisotropy Domains, Loop Domains, and Reorientation, Chem. Rhys. Lett., 14, p. 503 (1972). 

H. Gruler and G. Meier, Electric Field Induced Deformations in Oriented Liquid Crystals of the Nematic Type, Mol. Cryst. andLiq. Cryst, 16, p. 299 (1972). 

For a single, two-dimensional liquid crystal hpid bilayer, or a membrane, in a smectic A-like state, the director field is represented by the membrane normal n. Fiexoelectricity is then defined as a curvature-induced area membrane polarization, or, conversely, as an electric field-induced membrane curvature. Lipids and proteins are oriented parallel to each other along the local membrane normal in the flat state. A curvature of the membrane surface leads, indeed, to a splay type deformation of the molecular local director, with a splay vector S = (si - - S2)n, while a bend deformation along the membrane normal is not allowed because there is no third dimension. Then, obviously, the only polarization component points along the membrane normal. 

Figure 10.24. Molecules with a bent molecular core are expected to show a high flexoelectric coefficient. Thus, an electric field-induced bend deformation may be suitable to generate a lattice of edge dislocations, thereby leading to a bend grain boundary phase.

In compliance with their intrinsic elastomeric behavior die mechanical characteristics of DEs correspond to a mechanical spring. The electrically field-induced contraction of the elastomer between the electrodes causes a material movement in lateral direction due to its incompressibility. Obviously the actuator dimensions (electrode area, total height), the deformation ratio, and the boundary conditions (passive surroundings, contact area, and clamps) define the resulting compliance. According to the required conditions we measure the deformation dependent force. 

At present, at least three types of steady-state dielectrically driven pattern are known for nematics. The electric-field-induced periodic bend distortion in the form of parallel stripes has been observed in a homeotropi-cally oriented layer of 5-CB ( a= 13) in the presence of a stabilizing magnetic field [75, 76]. The stripes with a wavevector q were parallel to the electric field E and stationary at low fields. It was shown that a stable periodic pattern of the director minimizes the free energy of the cell when the ela.stic moduli and A 33 are similar to each other. In these experiments the Frederiks transition is of first order, the nonde-formed and deformed areas coexist at a given voltage, and the front between them may propagate along the direction y perpendicular to both fields [77]. 

The converse piezoelectric effect, when electric field induces mechanical deformation, has special importance because of its influence on the electro-optical responses they are mainly imwanted since they result in misalignment, but with clever design can be used to re-heal alignment. It is interesting to note that in the audio frequency ranges, the vibrations result in audible acoustic effects, implying their possible use in electromechanical transducers. ... 

F. J. Kahn. "Electric Field-Induced Orientational Deformation of Nematic Liquid Crystals Tunable Birefringence. Appl. Phys. Lett., Vol. 20, p. 199 (1972). 

U.D. Kini, Magnetic and Electric Field Induced Periodic Deformations in Nematics, J. de Physique II, 5, 1841-1861 (1995). 

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