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Active colloids self-propel in the fluid and their motion is driven by either internal or external
energy. Electrokinetics is combination of both electrophoresis (motion of particles) and electroosmosis
(motion of fluid) occurs upon application of electric field. Electokinetics in colloidal
systems represents a broad and interesting class of out-of-equilibrium effects, caused by the external
electric field that acts on spatially separated charges. Usually, the colloids are dispersed in
an isotropic electrolyte such as water. In this seminar, we discuss novel features of the dynamic
out-of-equilibrium behavior of colloids when the disperse medium is an anisotropic electrolyte,
namely, a nematic liquid crystal. Because of orientational order, nematics show anisotropy of
properties such as electric conductivity and dielectric permittivity. When the electric field is
applied to a nematic with a director that varies in space, anisotropy causes the ionic charges to
separate in space. The density of space charge is proportional to the applied field, director gradients,
anisotropy of conductivity and permittivity. In other words, separation of charges is defined
by the properties of the liquid crystal electrolyte rather than by the properties of the solid component
as in the case of isotropic electrolytes. The separated charges move in the electric field,
creating electro-osmotic or electrophoretic effects with velocities proportional to the square of the electric field. We use nematic cells with photopatterned director field to illustrate the principle
of the liquid crystal-enabled electrokinetics and its advantages, such as steady character of
flows, control of vorticity, and the ability to transport colloidal particles of any type (solid, liquid,
gaseous). We also demonstrate that the colloidal particles can be electrophoretically active
even in a uniform nematic cell, provided their surface induces local dipole-like director distortions.
Such a particle will move even if the electric conductivity is isotropic, being supported
solely by the dielectric anisotropy. We demonstrate that the change in the sign of anisotropy
causes reversal of the particle motion. Finally, we also demonstrate that the electric field can
produce particle-like propagating solitary waves representing self-trapped “bullets” of oscillating
molecular director. These director bullets lack fore-aft symmetry and move with very high
speed perpendicularly to the electric field and to the initial alignment direction. The bullets are
true solitons that preserve spatially confined shapes and survive collisions. |