UC San Diego

This thesis is an experimental work, investigating how active matter can control the assembly of soft materials. We design a novel experimental system, combining sticky colloids sedimented on the bottom surface of a glass capillary with a bath of motile {\it E. coli}. The colloids diffuse and stick together, assembling into large, quasi - 2D aggregates. Motile {\it E. coli} generate forces and flows in the surrounding media: an active bath which injects energy into the system through mechanical work. In the active bath, aggregates exhibit a a persistent clockwise rotation, leading to a non-conventional aggregation mechanism. These aggregates form structures which are not accessible via conventional aggregation in a thermal bath. Aided by numerical simulation of spinning, sticky beads, we elucidate that the rotation and folding of aggregates is the salient feature driving the structural differences, and the activity of the bath controls the phase diagram of aggregation. Further experiments indicate that the bacteria collide with and then swim through the aggregates; additionally, the direction of rotation of the aggregates is correlated with the direction of the circular trajectories made by the {\it E. coli} bacteria. Based on these insights, we propose a simple model for the swimmer-aggregate interactions, and propose further experiments to test its validity. As a whole, this work constitutes a proof of concept that active matter can be harnessed to direct the assembly of soft materials. The experiments presented in this thesis lay the groundwork for the development of a new class active, soft materials, whose structure and mechanical properties are dictated by their assembly in an active bath.