Equation of State and Reentrant Collision of Active Dumbbells under Stiff Wall Interactions*

Active systems of energy-consuming components are persistently out of equilibrium. It is known that pressure is not always a state function for generic active matter. Torque interaction with confinement affects the distribution of active particles throughout the system. Thus, the mechanical pressure of anisotropic active particles depends on their microscopic interactions with a solid wall. We perform numerical simulations of self-propelled dumbbells to explore how variations in the wall stiffness influence the mechanical pressure of dry active matter. In contrast to previous findings, we find that mechanical pressure can be independent of the wall interaction, even in the presence of intrinsic torques for dumbbells. Particularly, the dependency of pressure on the wall stiffness vanishes when the stiffness is above a critical level. In such a limit, the dynamics of dumbbells near the walls are randomized due to the large torque experienced by the dumbbells, leading to the recovery of pressure as a state variable of density. We also observe anomalous collision dynamics at high wall stiffnesses induced by inertial effects, in which a dumbbell exhibits multiple collisions with the wall in a very short time before leaving the wall indefinitely.