Wall stiffness effects on the pressure of active rods

The near-wall pressure of active systems could play a major role in their morphological behavior and assembly states under confinement. Previous studies showed that, except for very specific systems (e.g., without any torque interactions between particles and the wall such as spherical particles), the strength of particle-wall interaction affects the mechanical pressure of active fluids. However, to what extent this dependence is applicable has not been investigated yet. We use Langevin dynamics simulations to study swim pressure of 2D dry active systems of overdamped self-propelled rods with negligible noise. To elucidate effects of wall stiffness on pressure, we introduce a mobile wall to the middle of the simulation box, which exerts asymmetric harmonic repulsions to rods on different sides. By tracking the wall movement and calculating the near-wall pressure for various wall stiffness, we find that the pressure-stiffness dependence is destroyed as wall stiffness increases, suggesting that pressure can reinstate an equation of state at this limit. These findings provide new insight into active pressure and could improve our prediction for the dynamical behavior of confined active systems.