Liquid-crystal-enabled electrophoresis of spheres in a nematic medium with negative dielectric anisotropy.

We describe electrophoresis of spherical dielectric particles in a uniformly aligned nematic medium with a negative dielectric anisotropy. A spherical particle that orients the liquid crystal (LC) perpendicularly to its surface moves under the application of a uniform direct current or alternating current electric field. The electric field causes no distortions of the LC director far away from the sphere. Electrophoresis in the nematic LC shows two types of nonlinearity in the velocity versus field dependence. The velocity component parallel to the applied electric field grows linearly with the field, but when the field is high enough, it also shows a cubic dependence. The most interesting is the second type of nonlinear electrophoresis that causes the sphere to move perpendicularly to the applied field. This perpendicular component of velocity is proportional to the square of the field. The effect exists only in an LC and disappears when the material is melted into an isotropic fluid. The quadratic effect is caused by the dipolar symmetry of director distortions around the sphere and is classified as an LC-enabled electrophoresis (LCEEP). The nonlinear electrophoretic mobility of particles in LCEEP offers a rich variety of control parameters to design three-dimensional trajectories of particles for microfluidic and optofluidic applications.