Switchable dielectrophoresis of defect-free droplets in an anisotropic liquid crystal medium

Abstract

Dielectrophoresis (DEP) is widely used in nanoscience and biology to control small particles but its applicability is significantly limited by its one-way impetus characteristics along the square field gradient (∇E²) direction, that is, DEP force, F_DEP ∼ ∇E². Here, switchable DEP (SDEP) using the anisotropic property of a nematic medium is demonstrated; F_DEP does not need to be parallel to ∇E² but is arbitrarily changeable depending on the permittivity tensor orientation of the medium. To effectively demonstrate the SDEP phenomenon, isotropic droplets with infinitesimal surface anchoring in a nematic medium are introduced, in which topological defects of the nematic medium around dispersed objects are effectively eliminated. The experimental behaviours are well explained by theoretical and simulation results. To emphasize the applicability of SDEP, switchable arrays of isotropic droplets and an isotropic pocket carrier system containing micro-particles are demonstrated. The results reveal a new dimension of DEP and provide a novel approach for manipulating nano- or micro-materials in colloids.