Polymer Cholesteric Liquid Crystal Flake Reorientation in an Alternating-current Electric Field

274 LLE Review, Volume 100 Introduction The motion and orientation of nonspherical particles suspended in a host fluid and subjected to an alternating-current (ac) electric field have been well studied theoretically. Because the dielectric properties of the two phases of the suspension differ, charge will start to accumulate at the particle–fluid interface due to Maxwell–Wagner polarization. This charge accumulation induces a dipole moment, which is acted on by the electric field, causing the particle to reorient. Basic electromagnetic theory predicts that a particle in an electric field will orient in the direction corresponding to the lowest energy so as to minimize the potential energy of the system. Typically this requires a dielectric nonspherical particle to align its longest axis parallel to the external field.1 Because the minimum energy orientation of the particle depends on the shape, the dielectric properties of the host fluid and particle, and the frequency of the ac field, it is possible that a particle will have varying stable orientations at different frequencies. Schwartz et al.2 described the orientation of particles in an ac electric field and considered the conditions for particles with anisotropic dielectric properties or with additional membranes or layers, which is of particular use in the biological sciences. Okagawa et al.3 derived similar equations for a uniform dielectric particle but included the effect of shear flow on the particle motion. Jones did further work in the field of particle electromechanics, including phenomena such as electrophoresis and dielectrophoresis,4 and a comprehensive review of electromechanical behavior of particles was written by Gimsa.5 In addition to conducting experiments with biological particles,6 Miller and Jones investigated highly dielectric particles, such as titanium dioxide.7 Bostwick and Labes also performed similar experiments using platelets of crystalline nafoxidine hydrochloride.8 In this article we report on the orientation of highly dielectric polymer cholesteric liquid crystal (PCLC) particles (flakes), suspended in a low-viscosity host fluid. Though much of the theory on particle electromechanics is well supported by experimental work in the biological sciences, relatively little work has been done to Polymer Cholesteric Liquid Crystal Flake Reorientation in an Alternating-Current Electric Field