Soliton-induced liquid crystal enabled electrophoresis

Abstract

The manipulation of particles by a uniform electric field, known as electrophoresis, is used in a wide array of applications. Of special interest is electrophoresis driven by an alternating current (ac) as it eliminates electrode blocking and produces a steady motion. The known mechanisms of ac electrophoresis require that either the particle or the surrounding medium is asymmetric. This asymmetry is usually assured before the field is applied, as in the case of Janus spheres. We report on a mechanism of ac electrophoresis, in which the symmetry is broken only when the field exceeds some threshold. This mechanism is rooted in the nature of the electrophoretic medium, which is an orientationally ordered nematic liquid crystal. Below the threshold, the director field of molecular orientation around a spherical particle is of quadrupolar symmetry. Above the threshold, the director forms a polar self-confined perturbation around the inclusion that oscillates with the frequency of the applied field and propels the sphere. The director perturbations are topologically trivial and represent particlelike solitary waves, called "director bullets"or "directrons."The direction of electrophoretic transport can be controlled by the frequency of the field. The ac directron-induced liquid crystal enabled electrophoresis can be used to transport microscopic cargo when other modes of electrophoresis such as induced charge electrophoresis are forbidden.