Defect propelled swimming and interactions of nematic colloids for microrobotics
April 22nd, 2024
Nematic liquid crystals (NLCs) are highly non-linear fluids that have elastic responses that resist nematogen rearrangement and high-energy defect sites at which nematogen order is lost. Generally, the field of nematic colloids seeks to develop control over these elastic responses and defect structures to tailor colloidal interactions. We have been studying ferromagnetic disk colloids rotated by an in-plane magnetic field in nematic liquid crystals. The disk diameter and rotation rate are sufficiently slow that colloid inertia is negligible. In Newtonian fluids, these colloids rotate without translation. However, in NLC, the colloids’ anisotropic defect structure and the NLC’s elastic response generate broken symmetries that propel colloid translation. For patchy, rough colloids, a defect loop which forms on the disk undergoes periodic defect pinning, release, and contraction. This periodic defect motion generates a swim stroke that powers colloidal swimming. Changes in defect configuration with rotation rate provide a steering mechanism. In addition to this swimming motion, colloid shape and surface chemistry generate long-ranged emergent interactions with neighboring passive colloids in quasi-static settings. Furthermore, the non-linear response of the nematic fluid host allows pair interactions among rotating disks that differ strikingly in range and form from their static counterparts. These interactions provide a rich toolkit for reconfigurable materials assembly and open important fundamental questions regarding swimming at low Reynolds number in NLC.