Steering Active Emulsions with Liquid Crystals.

Colloids dispersed in liquid crystals (LCs) diffuse preferentially along the LC director because this direction of displacement generates the lowest hydrodynamic drag. In this article, we report on the active transport of micrometer-sized nematic droplets of 4'-pentyl-4-biphenylcarbonitrile (5CB) propelled through a continuous LC phase formed from aqueous solutions of disodium cromoglycate (DSCG) by Marangoni stresses (generated through the addition of sodium dodecyl sulfate (SDS)). We observe the nematic droplets to exhibit motion guided by the continuous LC phase, but in contrast to passive diffusion, the LC droplets move preferentially in a direction perpendicular to the continuous-phase LC director. Our results suggest that the LC droplets, with internal symmetry broken by the Marangoni flow, interact through orientation-dependent van der Waals forces with the LC continuous phase, biasing the orientation of the droplets and the direction of propulsion orthogonal to the far-field director of the continuous LC phase. This proposal is supported by measurements of the orientations of droplets of 5CB and 4-ethoxy-4'-(6-acryloyloxyhexyloxy) azobenzene (RM257) polymerized in a preradial director configuration, which reveal the polymerized droplets to adopt orientations that are biased toward the perpendicular of the far-field DSCG director. Additionally, we demonstrate that preferential motion parallel to the continuous-phase LC director is recovered when using self-propelled isotropic oil droplets. We also observe periodic changes in the instantaneous velocities of LC droplets. We show the changes to correlate with the formation and detachment of satellite droplets, consistent with the solubilization of the nematic oil into surfactant assemblies near the trailing edge of the droplets and their accumulation near a stagnation region downstream of the droplet. Overall, our results provide fundamental insights into ways in which LC ordering can change the dynamics of active colloidal systems and hint at principles by which the motion of active colloids can be steered.