Topological constraints in a microfluidic platform

Abstract

Microfluidics has evolved as a major technological platform for biotechnology, material science and related fields. In virtually all of the areas of application, the flowing matrix is an isotropic fluid. However, replacing the typically isotropic fluid with an anisotropic liquid crystal opens up avenues beyond the viscous-dominated isotropic microfluidics. Especially, the material anisotropy of the flowing LC matrix and the consequent incorporation of topological constraints within the microfluidic device offer smart capabilities ranging from tunable flow-shaping to flexible micro-cargo concepts. The key to such capabilities lies in exploiting the possible topological constraints offered by the microfluidic confinement. As an example, we shall demonstrate how long-range ordering and consequent anisotropy in liquid crystals (LCs) could be utilised to devise a novel route to guided transport of microscopic cargo on ‘soft rails’, i.e. topological defect lines (disclinations). We create, position and navigate disclination lines within the LC matrix by tuning the coupling between flow and LC orientation. As model cargo elements, we have used isolated or self-assembled chains of colloidal particles, and demonstrated the broader capability of this method by transporting aqueous droplets on the defect lines. Topological constraints in combination with flow-director coupling thus endow LC microfluidics with features distinct from its isotropic counterparts.