Abstract:

Swimming, self-organization, and collective behaviors of active and driven colloids have implications in nature and applications to reconfigurable materials. Here, we focus on swimming or displacement made possible by the complex topologies of defects within nematic liquid crystals (NLCs).

In Newtonian fluids, colloidal swimming is challenging; broken symmetries or non-reciprocal forcing are needed to achieve a net displacement, given the reversibility of flows in inertia-free regimes. NLCs, made of elongated nematogens with highly non-linear elastic free energy, offer degrees of freedom that change this scenario. NLCs store elastic energy in non-uniform director fields, spontaneously generate topological defects, and have complex visco-dynamic responses. Recently, rotated isotropic spherical colloids have been shown to translate or swim in NLC. This translation under highly symmetric forcing arises through broken symmetries in the viscous stresses which depend on the local director field. Distortions in the director generated by the flow result in net forces on the sphere.

In this thesis, we develop the concept of a topological flagellum, a defect in the NLC that undergoes non-reciprocal rearrangement near the colloid and aids in its translation. Ferromagnetic disk colloids, short circular cylinders of finite height, are rotated in an external magnetic field in an NLC, which generates translation. For disks with uniform anchoring, swimming occurs via mechanisms like those for the rotating sphere. For disks with hybrid anchoring, however, a companion topological defect near the disk emerges which undergoes a non-reciprocal, periodic, elongation, sweeping and contraction cycle. The disk and defect together periodically store and release elastic energy in a highly directed manner. We identify the defect to be a thick, non-singular twist wall, and demonstrate that the sweeping defect occurs via a topological instability in which the sense of twist over the disk changes handedness. We show that defect motion in the super-critical regime aids propulsion by two mechanisms. The sweeping twist wall generates material flows and net viscous shear forces via rotation of nematogens as sweeping occurs. After sweeping, the defect contracts on the disk’s edge, driving rotation that aids propulsion. Defect-driven propulsion provides strategies to create soft, reconfigurable micro-machines powered by the intrinsic physics of structured fluids.

Zoom Information:

Meeting ID: 988 3278 8468

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