Qi Xing Zhang | Searcharxiv

arXiv2026

A sweeping twist defect as a topological flagellum that drives colloid motion

Nematic liquid crystals can dramatically reconfigure under dynamic forcing, providing exciting opportunities in active matter. Here, we study a hybrid disk colloid rotated by an external field which generates a dynamic companion topological defect. The disk moves faster when the defect sweeps across the disk's face. We identify the defect as a non-singular twist wall, characterize the twist energy landscape, and identify the sweeping motion as a topological instability. As the defect sweeps, it reverses the handedness of twist and lowers the free energy in the fluid in the gap above the disk. Landau-de Gennes modeling shows that the sweeping wall behaves as a propagating director texture: the director field is nearly stationary in the wall frame, while nematogens rotate locally as the wall passes. The nematogens' rotation generates a viscous stress on the surface of the disk that hastens its propulsion. Thus, the defect acts as a flagellum that powers colloid swimming, providing an example of a dissipative topological structure whose dynamics can be harnessed to perform useful work.

cond-mat.soft↗

arXiv2025

Scallop Theorem for Swimming in Anisotropic Fluids

In isotropic fluids like water, micrometer-scale swimmers have evolved swim strokes to translate despite their tiny size. As described by Purcell in his Scallop Theorem, reciprocal motions, like those performed by a scallop, cannot drive swimming when inertial effects are absent, as is typical at micrometer length scales. Thus, microswimmers have evolved complex structures that can perform non-reciprocal swim strokes or body displacements to generate motion. Microswimmer dynamics in structured fluids differ fundamentally from those in isotropic fluids because of their inherent asymmetry. The orientation of elongated constituents and the topological defects that spontaneously form near microswimmers provide broken symmetries, even atequilibrium. This is sufficient for the dynamic disturbance of even the simplest isotropic swimmers to generate propulsion. We combine experiments on magnetically rotated colloids in nematic liquid crystals with analytic non-equilibrium solutions to formulate propulsion strategies for microswimmers in nematic fluids and determine how swimming velocity depends on the rotation rate, materials parameters, and forcing regimes. For example, we find that micro-scale spherical colloids swim effectively under continuous rotation and under reciprocal forcing.Thus, swim strokes that are ineffective in isotropic fluids are highly effective in nematic liquid crystals. In light of these observations, the Scallop Theorem is extended for structured fluids.

cond-mat.soft↗