Active pressure of bacterial suspensions

For an equilibrium system, thermodynamic pressure is considered as a fundamental state variable of the system, which always equals to mechanical pressure, i.e., force per unit area acting on confining walls. In contrast, for an active system out-of-equilibrium such as bacterial suspensions, mechanical pressure is no longer a state variable, which may depend on the stiffness and shape of confining walls. Here, by combining optical tweezers with biochemical engineering technique, we create quasi-two-dimensional bacterial suspensions and systematically study active pressure exerted by swimming E. coli on confining walls of different geometries. In particular, we measure the pressure of bacterial suspensions on V-shaped walls of different angles. We find that the active pressure is a function of the angle of the walls: a sharper angle leads to a stronger pressure. In addition, the fluctuation of pressure increases with decreasing angle. We construct a simple model based on the wall-induced alignment of bacteria to quantitatively explain our observations. Our study provides benchmark experiments for characterizing the mechanical pressure of bacterial suspensions and sheds new light on the nonequilibrium statistical principle of active fluids.