Boundary aggregation of rod-shaped active agents.

A characteristic feature of self-propelled active particles is their tendency to accumulate near confining boundaries. In this study, we investigate this form of boundary aggregation by conducting experiments on surface-swarming bacterium, Vibrio alginolyticus, in microchannels and performing LAMMPS simulations of soft active rods confined in straight and annular channels. Both systems display non-conventional shear layers compared with passive flows and reveal a sharp transformation in the effective channel width induced by boundary aggregation. We analyze the self-organization process underlying this aggregation and quantify the interplay among several key dynamical and structural characteristics, including agent density, speed, orientation, alignment, and boundary curvature. Several identified features highlight intriguing parallels between boundary aggregation and motility-induced phase separation (MIPS), such as an inverse relationship between speed and density, a long reorientation time, and a positive feedback mechanism.