E. Fodor

Microscopic efficiency sets the kinetics and structure of active fluids

In contrast with systems driven by an external field, the energy injection in active matter is local and independent for each particle, opening the possibility of a phase separation even with purely repulsive interactions. While such a phenomenon has been studied extensively, understanding how the microscopic energy fluxes control the emerging collective behavior, and its connection with entropy production [1, 2], has remained an elusive goal.

Based on methods of stochastic thermodynamics, we define a particle based efficiency as the proportion of injected power which effectively leads to collisional slowing down of the dynamics. We demonstrate that there exist generic relations between such an efficiency and transport properties quantified via the effective diffusion and mobility of an internal tracer. Moreover, we show that the spatial profile of efficiency controls the structure of a phase separated state by setting the form of the interface between dilute and dense phases. We also discuss how the instantaneous efficiency reveals failures in compact structures, such as fractures and moving defects.

These recent findings shed a new light on the control of active fluid properties by microscopic efficiency. It opens the door to the design of new active systems based on monitoring locally internal energy fluxes.