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First direct measurement of Pressure in an active liquid

Developing thermodynamics for out of equilibrium systems of dissipative particles has been a long-lasting effort driven by the impressive success of equilibrium thermodynamics. Recently, defining pressure in systems of active particles, the specificity of which is to turn the injected energy into directed motion, has attracted a lot of attention. While at equilibrium, mechanical, hydrodynamic and thermodynamic pressures are all equal quantities, which inherit their mutual properties, it is a priori not the case in active liquids. Probing experimentally the mechanical pressure exerted by a set of respectively passive isotropic and active polar disks onto two different flexible one-dimensional membranes, we have shown that the mechanical pressure, in the active case, strongly depends on the membrane in use, and is thus not a state variable. The underlying dynamical mechanisms are also responsible for a new form of instability.

G. Junot, G. Briand, R. Ledesma-Alonso, and O. Dauchot, “Active vs. Passive Hard Disks Against a Membrane: Mechanical Pressure and Instability”, PRL 119, 243459-2 (2017). Editor’s suggestions.

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