Gas–Solid Phase Separation of Active Brownian Particles Under Confinement of Hard Walls

By means of computer simulations, we have investigated the gas-solid phase separation of active Brownian particles (ABPs) under the confinement of two hard walls, distinct from the gas-liquid phase separation typically seen in bulk systems. Our results show that the distance (D) between the hard walls plays a crucial role. Increasing D may facilitate the formation of gas-solid phase separation perpendicular to the hard walls, while decreasing D may suppress such phase separation. Interestingly, when D is decreased further and the lateral system size is increased accordingly to maintain a constant volume, a new reoriented phase separation pattern in the system emerges, i.e., the gas-solid phase coexistence can be found in those layers parallel to the inner surfaces of two hard walls. These intriguing findings illustrate how ABPs can achieve simultaneous localization and crystallization under imposed boundary confinement, thereby fundamentally altering the pathway of phase separation. Also, such understanding may provide a valuable pathway for optimizing the design of systems full of active matters such as micro-robotics or targeted delivery platforms.