UC Berkeley

Active matter, composed of agents that consume energy at the microscopic scale, exhibits a wealth of collective behaviors that transcend traditional equilibrium thermodynamics. An example of this behavior is motility-induced phase separation (MIPS), where self-propelled particles assemble into dense and dilute phases in the absence of attractive interactions. While the bulk phase behavior of MIPS is becoming increasingly well-understood, the interfaces separating these active phases remains a subject of significant debate. Surprising observations, such as the measurement of negative surface tensions and the emergence of unique ”bubbly” phase separation in two dimensions, challenge our traditional understanding of interfaces. This thesis is devoted to resolving these outstanding questions surrounding active interfaces. We first develop a generalized capillary-wave theory for active systems by deriving an equation of motion for active interfaces and extracting out a non-equilibrium surface tension informed by the microscopic particle dynamics. We are able to extend this generalized capillary-wave theory to chiral systems, predicting (and verifying) the presence of odd-surface flows when spatial parity symmetry is broken. We then investigate nucleation and coarsening in active matter, providing a route to calculating finite-size corrections to binodals and extending classical coarsening theories to non-equilibrium systems. Finally, we examine the origins of bubbly phase separation in two-dimensional MIPS, revealing that appreciable formation of bubbles depends on the interplay between hexatic order and translational noise. Collectively, this work aims to establish a more systematic theoretical foundation for understanding the structure, stability, and dynamics of interfaces in active matter.