Oscillating edge current in polar active fluid

Abstract

Dense bacterial suspensions exhibit turbulent behaviour called ``bacterial turbulence''. The behavior of the bulk unconstrained bacterial turbulence is described well by the Toner-Tu-Swift-Hohenberg (TTSH) equation for the velocity field. However, it remains unclear how we should treat boundary conditions on bacterial turbulence in contact with some boundaries (e.g. solid walls). To be more specific, although the importance of the ``edge current'', the flow along the boundary, has been demonstrated in several experimental studies on confined bacterial suspensions, previous numerical studies based on the TTSH equation employ non-slip boundary conditions and do not seem to describe properly the behavior of bacteria near the boundaries. In this study, we impose a slip boundary condition on the TTSH equation to describe the bacterial motion at boundaries. We develop a method to implement the slip boundary condition. Using this method, we have successfully produced edge current and discovered that the direction of the edge current temporally oscillates. The oscillation can be attributable to the advection term in the TTSH equation. Our work demonstrates that boundary conditions could play an important role in the collective dynamics of active systems.