Constraint dependence of pressure on a passive probe in an active bath.

Mechanical pressure in active matter is generally not a state variable and possesses abnormal properties, in stark contrast to equlibrium systems. We here show that the pressure on a passive probe exerted by an active fluid even depends on external constraints on the probe by means of simulation and theory, implying that the mechanical pressure is not an intrinsic physical quantity of active systems. The active mechanical pressure on the passive probe significantly increases and saturates as its elastic constraint (realized by a trap potential) or kinematic constraint (realized by environmental friction) strengthens. The microscopic origin for the constraint-dependent pressure is that the constraints influence the probe dynamics, and hence change the frequency and intensity of the collisions between the probe and active particles. Our findings not only greatly advance the understanding of active mechanical pressure but also provide a new way to in situ tune it.