Liquid Crystals-Enabled AC Electrokinetics

Abstract

Phenomena of electrically driven fluid flows, known as electro-osmosis, and particle transport in a liquid electrolyte, known as electrophoresis, collectively form a subject of electrokinetics. Electrokinetics shows a great potential in microscopic manipulation of matter for various scientific and technological applications. Electrokinetics is usually studied for isotropic electrolytes. Recently it has been demonstrated that replacement of an isotropic electrolyte with an anisotropic, or liquid crystal (LC), electrolyte, brings about entirely new mechanisms of spatial charge formation and electrokinetic effects. This review presents the main features of liquid crystal-enabled electrokinetics (LCEK) rooted in the field-assisted separation of electric charges at deformations of the director that describes local molecular orientation of the LC. Since the electric field separates the charges and then drives the charges, the resulting electro-osmotic and electrophoretic velocities grow as the square of the applied electric field. We describe a number of related phenomena, such as alternating current (AC) LC-enabled electrophoresis of colloidal solid particles and fluid droplets in uniform and spatially-patterned LCs, swarming of colloids guided by photoactivated surface patterns, control of LCEK polarity through the material properties of the LC electrolyte, LCEK-assisted mixing at microscale, separation and sorting of small particles. LC-enabled electrokinetics brings a new dimension to our ability to manipulate dynamics of matter at small scales and holds a major promise for future technologies of microfluidics, pumping, mixing, sensing, and diagnostics.

Electrorotation of semiconducting microspheres

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Active transformations of topological structures in light-driven nematic disclination networks

“…In equilibrium, nematics can form a uniform state or static topological defects due to thermal fluctuations or geometric frustrations ( 10 , 16 ). When a nematic is driven by externally ( 17 , 18 ) or internally injected energy ( 12 ), the system becomes out of equilibrium or active, and a chaotic flow is developed, accompanied by persistent nucleation and annihilation events of topological defects ( 19 , 20 ). Understanding topological structures has become the key to understanding active nematics ( 12 , 14 , 20 ).…”

Self-Assembly of Aqueous Soft Matter Patterned by Liquid-Crystal Polymer Networks for Controlling the Dynamics of Bacteria

“…We used the recently invented photopatterning technique based on the plasmonic metamasks. − The mask is made of a thin layer of aluminum film with a spatially varying orientation of nanoslits. A nanoslit has an aperture of length 220 nm and width 100 nm.…”