Aggregation of colloidal particles in a non-equilibrium backflow induced by electrically-driven reorientation of the nematic liquid crystal

Abstract

We study the dynamics of anisotropic aggregation of spherical colloidal particles with dipolar interactions, dispersed in a nematic liquid crystal. The director is perpendicular to the surface of spheres so that each particle creates a dipolar distortion when placed in a uniformly aligned planar nematic slab. A backflow of the nematic caused by periodic voltage pulses, propels the spheres in a bidirectional fashion, so that the dipoles of one polarity move in the direction antiparallel to the dipoles of opposite polarity. The dipoles experience inelastic collisions, head to tail and head to head. The head-to-tail collisions of similarly oriented dipoles result in linear aggregation parallel to the backflow, while the head-to-head collisions lead to aggregation in a transversal direction. The two flows are separated by an impact parameter that controls the frequency of head-to-head collisions and thus the resulting shape of aggregates, their anisotropy and fractal dimension. We discuss in detail the numerous forces of elastic, electrophoretic and dielectrophoretic nature, as well as gravity forces, acting on colloidal spheres suspended in an electrically driven slab of a nematic liquid crystal. The proposed “nematic collider” offers opportunities in the quantitative studies of non-equilibrium processes such as anisotropic aggregation and jamming, as well as in the development of systems capable of self-assembly through anisotropic interactions of the building units.