Title: Active Matter and Choreography at the Colloidal Scale
Description: In this thesis, I present numerical simulations that explore the applications of self-propelled particles to the field of self-assembly and to the design of `smart' micromachines. Self-propelled particles, as conceived of here, are colloidal particles that take some energy from their surroundings and turn it into directed motion. These non-equilibrium particles can move persistently for long times in the same direction, a fact that makes the behavior of dense and semi-dilute systems of these particles very different from that of their passive counterparts. The first section of this thesis deals with the interactions between passive components and baths of hard, isotropic self-propelled particles. First, I present simulations showing how the depletion attraction can be made into a short ranged repulsive, or long ranged attractive interaction for passive components with different geometries in a bath of self-propelled particles, and show how the form of these interactions is consistent with how active particles move near fixed walls.

In the next chapter, a rigid filament acts as a flexible wall that engages in a feedback loop with an active bath to undergo repeated folding and unfolding events, behavior which would not occur for a filament in a passive environment. The subsequent chapters deal with self-propelled particles that have long ranged and anisotropic interactions. When the orientations of active particles are coupled, they can undergo remarkable collective motion. While the first chapter in this section begins with a discussion of how active disks interacting via an isotropic potential consisting of a long ranged repulsion and short ranged attraction self-assemble into living clusters of controllable size, I show how replacing the disks with anisotropic dumbbells causes these clusters to rotate coherently. In the last chapter, I show that weakly screened active dipoles form lines and clusters that move coherently. These particles can become anchored to the surface of a passive charged colloid in various ways that lead to two different kinds of active motion: rotations of a corona of dipoles around the colloid, and active translation of the colloid, pushed by a tail of dipoles.

Finally, a mixture of many charged colloids and dipoles can reproduce the swarming behavior of the pure dipoles at a larger length scale with coherent motion of the colloids. These are all examples of how activity is a useful tool for controlling motion at the micro-scale.