Pressure exerted by confined active particles

Mechanical pressure, defined as the force applied perpendicular to the surface of an object per unit area, takes dramatically different forms in and out of equilibrium [1]. To experimentally measure the pressure exerted by an out of equilibrium system, we confine self-propelled particles in a negative dielectrophoretic trap. Knowing the potential profile, the density profile reveals the pressure. In equilibrium, pressure is exclusively governed by bulk properties such as density and temperature. In contrast, when the particles are activated, using either an in-plane rotating magnetic field or light-activated self-phoretic particles, we find that the force exerted on the confining trap boundaries depends on the shape of the boundary dielectrophoretic force fields.

[1] Solon, A.P., Fily, Y., Baskaran, A., Cates, M.E., Kafri, Y., Kardar, M. and Tailleur, J., 2015. Pressure is not a state function for generic active fluids. Nature Physics, 11(8), p.673.