Anisotropic Colloidal Assembly: Kinetics, Shape Complementarity, and Field-mediated Propulsion.

Abstract

We use confocal microscopy and particle tracking methods to study the binding kinetics and equilibrium thermodynamics of suspensions of anisotropic lock and key colloids that self-assemble in the presence of polyethylene oxide polymeric depletant. We find that specific lock-key bonds form by (1) the diffusion of a key into the lock dimple from bulk as well as (2) by the surface diffusion of a key particle that binds to the spherical surface of the lock and diffuses on its surface until binding to the lock dimple. We compare experimental results to a Smoluchowski diffusion-migration model and find quantitative agreement between both. We also perform equilibrium binding experiments with different sized key particles and find lower free energies of specific bond formation for key particles smaller than the dimple size implying that smaller keys have better overall binding affinity to the lock dimple than keys larger than the lock dimple. These results agree with previous modeling work that predicts optimized formation of specific lock-key bonds for spherical key particles smaller than the lock cavity radius. In the second part of the dissertation, we investigate the emerging propulsion of colloidal spheres in binary suspensions subject to a low frequency oscillatory electric field perpendicular to the plane of motion of the particles. The effect is switchable: particles revert back to normal diffusion upon turning off the electric field. The active motion of particles is attributed to unbalanced electrohydrodynamic flow. Large particle motion is characterized using confocal microscopy and particle tracking algorithms. We find that the mean-squared displacement of the particles is well fit by single-particle active motion model of mean-squared displacement. The propulsion speed of large particles increases with increasing applied voltage and decreases with increasing frequency of oscillation. We find that the short-time ballistic motion of a large particle depends on the number of small particle neighbors surrounding it, where particles with 2 and 3 small neighbors move fastest.