Self-organised polar and nematic structures at the gas-liquid interface of motility-induced phase separation

In a collection of active polar particles with purely steric interactions, a condensed (liquid) phase of active particles can co-exist with a dilute (gas) phase of active particles, in a phenomenon known as motility-induced phase separation (MIPS). Although both the liquid and gas phases are apolar, the interface is simultaneously polar and nematic, leading to interesting properties such as having a negative surface tension. By using a mean-field analysis of a system of active particles with discretized orientations in two dimensions, I obtain the interfacial profiles and elucidate various interfacial properties of the system under MIPS.