Pressure of bacterial suspensions on confining walls and freely moving chains

An active matter consists of a large number of self-propelling units capable of taking in, employing and dissipating energies. Different from equilibrium systems with detailed balance, active matter exhibits many unusual properties, highlighting the nonequilibrium nature of the system. In particular, mechanical pressure, a state variable in equilibrium systems, shows a nontrivial dependence on system boundary as well as specific measurement methods. Here, we experimentally study the mechanical pressure of bacterial suspensions by exploring the interactions between swimming Escherichia coli and confining walls of different shapes, as well as flexible chains of colloids linked by DNA in a quasi-two-dimensional space. We find that the pressure exerted by E. coli depends nonmonotonically on the apex angle of the V-shaped confining walls. We further characterize the interactions between E. coli and the freely-moving particle chains via imaging the fluctuation of the configuration of the particles and explore their dependence on the rigidity and the length of the chains. A hydrodynamic model is finally proposed to explain our results. Our study sheds light onto the mechanical properties of active matter and provides new understanding on the pressure exerted by swimming microorganisms.