Soliton-induced liquid crystal enabled electrophoresis

Abstract

Manipulation of particles by a uniform electric field, known as electrophoresis, is used in a wide array of applications. Of especial 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 are asymmetric. This asymmetry is usually assured before the field is applied, as in the case of Janus spheres. We report on a new mechanism of AC electrophoresis, in which the symmetry is broken only when the field exceeds some threshold. The new mechanism is rooted in the nature of 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 a 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.

Recent Progresses on Experimental Investigations of Topological and Dissipative Solitons in Liquid Crystals

“…It then carries and translates the particle by moving it through the nematic bulk. Similar phenomena were also reported by Li et al which was termed "directron-induced liquid crystal-enabled electrophoresis" [120].…”

“…This not only broadens the research and understanding of LCs, but also enhances our understanding of solitons in other physical systems. Furthermore, the solitons in LCs may even lead to novel phenomena, such as emergent collective motion of solitons [92,93], and applications, such as micro-cargo transport [102,103,120], optic processing [84,85], or fast LC displays [14]. We hope this brief review can arouse more researchers' interest in the field of solitons in LC systems.…”

Design of nematic liquid crystals to control microscale dynamics

“…The most important distinction is of a topological nature, as the solitons can be topologically protected (non-transformable into a ground uniform state by continuous deformation of the director) or unprotected, topologically equivalent to the uniform ground state. I first describe the current status of the so-called topological solitons and nematicons and then discuss the topologically trivial dissipative solitons, represented by the so-called director bullets (11)(12)(13)(14) or directrons (14).…”

“…Experimentally, dissipative solitons were realized in the form of electric current filaments in a 2D planar gas-discharge system (60). As often happens, nematics provide a fertile ground to realize entities that are difficult to form or to observe in other materials: very recently, various types of dissipative solitons driven by an electric field were experimentally realized in nematics (11)(12)(13)(14)61).…”

“…A more descriptive term is a "directron", introduced later by Li et al in Ref. (14), as it stresses the dual wave-particle character of the formation, in which the solitary wave of the director behaves as a particle in events such as collisions with other directrons. The first mathematical model of these structures has been proposed recently by Earls and Calderer (62) in the limit of linear approximation, which is a very promising step towards a complete nonlinear description.…”