Abstract:
In materials science, the ability to direct colloidal assembly and dynamically reconfigure the resulting structures is central to the development of adaptive functional materials. Nematic liquid crystals (NLCs) provide several mechanisms for achieving this control, including long-range nematoelastic interactions, topological defects, responsiveness to external fields, and the directed motion of externally driven colloids. NLCs consist of anisotropic molecules that exhibit long-range orientational order, described by the director field. Spatial variations in the director produce splay, twist, and bend deformations, each associated with an elastic free energy cost. Confining boundaries further prescribe preferred director orientations through surface anchoring. When incompatible boundary conditions cannot be accommodated by continuous director distortions, topological defects form as localized regions in which orientational order is reduced and the director becomes undefined. These defects create structured elastic energy landscapes that can trap, guide, and organize colloidal matter, making confined NLCs promising media for reconfigurable assembly and transport. Recently, rotationally driven spherical colloids have been shown to translate through NLCs. This motion arises because the polar particle–defect configuration and rotation-induced director distortions break spatial symmetry, allowing anisotropic viscous stresses to convert an applied torque into directed translation. This new finding also provides a new ground for colloidal enabled topological reconfiguration in NLCs.

In this thesis, I am advancing this field in two distinct studies. In the first study, I use maskless photopatterning to “print” spatially varying director fields and disclination networks. Homeotropically anchored magnetic colloids, each accompanied by a hedgehog defect, are actuated by a spatially uniform rotating magnetic field. The polar particle–defect configuration and anisotropic director–flow coupling convert particle rotation into translation. Within patterned director fields, particles move predominantly perpendicular to the local director, and reversing the direction of rotation reverses their translational motion. The programmed director therefore provides local guidance, while the uniform magnetic field supplies global actuation. Across four director patterns, particles traveled over distances exceeding 600 μm at speeds up to approximately 3 μm/s, without physical channels, localized field gradients, or individual particle control. Photopatterned disclination lines further act as elastic topological tracks that capture particles, confine transverse motion, and support reversible transport. Under stronger forcing, driven particles stretch
and reorganize the lines and recruit additional particles from the bulk, enabling defect mediated cargo collection and collective assembly, and topological transitions of the defects themselves.

In the second study, I use lithographically defined micropores with homeotropic anchoring to stabilize point defects in confined NLCs. In elongated, pillbox-shaped pores, the curved lobes create two energetically favorable locations for a radial hedgehog defect, separated by an elastic barrier near the pore waist. An applied direct-current electric field drives the defect between these locations, while the selected state remains stable after the field is removed. The switching threshold and migration dynamics depend on pore geometry and surface anchoring, demonstrating that confinement can be used to program the defect energy landscape. Reversible switching was observed throughout patterned pore arrays, with selected states retained for more than eight months. The polarity-sensitive migration cannot be explained by the conventional dielectric effect which would be quadratic in the
external field, and is consistent with additional coupling involving defect-associated space charge and flexoelectric polarization.

Together, these studies establish a general strategy in which surface geometry and anchoring encode an elastic landscape, external fields drive controlled reconfiguration, and nematic elasticity guides or preserves the resulting state. The work advances topological defects from passive features of ordered media to programmable components capable of switching, transport, memory, and materials assembly.

Zoom:
Link: https://upenn.zoom.us/j/7833623289
Meeting ID: 783 362 3289