Active Janus Motors in Acoustic Confinement

We analyze the active random motion of self-propelled Janus motors confined in an external, near-Harmonic acoustic trap. The external trap behaves as an “osmotic barrier” that confines the swimmers inside the trapping region, analogous to semipermeable membranes that confine passive Brownian particles inside a boundary. We discover that active particles exposed to a weak trap have a familiar Boltzmann-like probability distribution – analogous to that of passive, equilibrium Brownian suspensions – except with the thermal energy k_BT replaced by an intrinsic kinetic ‘activity’ scale of active matter. However, this equivalence breaks down for stronger traps, where the trap size becomes a macroscopic length scale that confines the run length of the microswimmers. From the swimmers’ restricted motion inside the trap, we calculate the unique mechanical ‘swim pressure’ generated by active systems originating from the force required to confine them by boundaries. Finally, we analyze the crossover from ballistic to diffusive behavior of active matter by tracking the explosion process of a dense active crystal.