Nonlinear electrophoresis of dielectric and metal spheres in a nematic liquid crystal.

Electrophoresis is a motion of charged dispersed particles relative to a fluid in a uniform electric field. The effect is widely used to separate macromolecules, to assemble colloidal structures and to transport particles in nano- and microfluidic devices and displays. Typically, the fluid is isotropic (for example, water) and the electrophoretic velocity is linearly proportional to the electric field. In linear electrophoresis, only a direct-current (d.c.) field can drive the particles. An alternating-current (a.c.) field is more desirable because it makes it possible to overcome problems such as electrolysis and the absence of steady flows. Here we show that when the electrophoresis is performed in a liquid-crystalline nematic fluid, the effect becomes strongly nonlinear, with a velocity component that is quadratic in the applied voltage and has a direction that generally differs from the direction of linear velocity. The new phenomenon is caused by distortions of the liquid-crystal orientation around the particle that break the fore-aft (or left-right) symmetry. The effect makes it possible to transport both charged and neutral particles, even when the particles themselves are perfectly symmetric (spherical), thus allowing new approaches in display technologies, colloidal assembly and separation, microfluidic and micromotor applications.

Inelastic collisions and anisotropic aggregation of particles in a nematic collider driven by backflow.

We design a nematic collider for controlled out-of-equilibrium anisotropic aggregation of spherical colloidal particles. The nematic surrounding imparts dipolar interactions among the spheres. A bidirectional backflow of the nematic liquid crystal in a periodic electric field forces the spheres to collide with each other. The inelastic collisions are of two types, head to tail and head to head. Head-to-tail collisions of dipoles result in longitudinal aggregation while head-to-head collisions promote aggregation in the transversal direction. The frequency of head-to-head collisions is set by the impact parameter that controls the resulting shape of aggregates, their anisotropy, and fractal dimension.