Anisotropy of average potential energy of passive plate in bacterial suspensions

Abstract We conduct optical-tweezers experiments to investigate the average potential energies of passive plates harmonically trapped in bacterial suspensions. Our results show that the mean potential energies along both the major and minor axes increase with bacterial concentration but decrease with trap stiffness. Notably, the average potential energy along the major axis consistently exceeds that along the minor axis. This discrepancy from equilibrium systems is primarily attributed to the distinct bacterial flow fields and direct bacterium–plate collisions near the major and minor axes, as evidenced by the higher orientational order around the plate along the major compared to the minor axis, despite identical bacterial densities in these regions. Our findings highlight the critical role of hydrodynamic interactions in determining the potential energy of passive objects immersed in an active bath.