Liquid crystals with patterned molecular orientation as an electrolytic active medium

Abstract

Transport of fluids and particles at the microscale is an important theme both in fundamental and applied science. One of the most successful approaches is to use an electric field, which requires the system to carry or induce electric charges. We describe a versatile approach to generate electrokinetic flows by using a liquid crystal (LC) with surface-patterned molecular orientation as an electrolyte. The surface patterning is produced by photo-alignment.In the presence of an electric field, the spatially varying orientation induces space charges that trigger flows of the LC. The active patterned LC electrolyte converts the electric energy into the LC flows and transport of embedded particles of any type (fluid, solid, gaseous) along a predesigned trajectory, posing no limitation on the electric nature (charge, polarizability) of these particles and interfaces. The patterned LC electrolyte exhibits a quadratic field dependence of the flow velocities; it induces persistent vortices of controllable rotation speed and direction that are quintessential for micro-and nanoscale mixing applications.

Experimental realization of the “lock-and-key” mechanism in liquid crystals

“…This suggests a reduced role for defect sharing, and that splay-matching plays a strong role in the particle docking. The localization of particles is related conceptually to particle sorting in the work by Peng et al, [20] in which splay deformation was created by photo-alignment. The complex, curved trajectory resembles the curved trajectory analyzed by Pires et al [6], and observed by Skarabot et al [21] and Nikkhou et al [8] for particles docking on a disclination line.…”

Electrokinetic flows in liquid crystal thin films with fixed anchoring

“…In order to study electrokinetic phenomena in realistic ranges of physical parameters, we have chosen a parameter set that corresponds to the experiments of Peng, et. al 12 . While we have shown good agreement between the numerical and experimental results, our comparison with experiments is limited by a number of factors: First, our numerical model is solved on a square domain of size ∼ 1 mm 2 or smaller, with the desired director pattern imposed throughout the cell, while the experiments are done in a chamber of dimensions 10 mm × 15 mm, with the desired pattern imposed on a 1 mm 2 region and uniform alignment on the remainder of the cell surface.…”

“…The field amplitude used numerically is the value of the nominal electrostatic field from the experiments of Peng, et. al 12 . Finally, the experiments use a mixture of MLC7026-000 and E7 liquid crystals at a ratio of 89.1:10.9 15 .…”

“…(54). Figure 8a shows, for example, a configuration with three disclinations of topological charge (-1/2, 1, -1/2), studied experimentally by Peng, et al 12 . In our numerical solution, Fig.…”

“…Such a configuration can be realized experimentally by placing a nematic thin film between two bounding surfaces on which a specific director orientation is specified through lithographic treatment of the surfaces 12 . We begin by considering a simple periodic configuration with wavevector perpendicular to the electrostatic field.…”