The Effective Temperature Concept Tested in an Active Colloid Mixture

TL;DR

This study demonstrates that an effective temperature concept can quantitatively describe phase behavior in a driven active colloid mixture, bridging nonequilibrium dynamics with equilibrium thermodynamics principles.

Contribution

It provides the first experimental and simulation evidence that effective temperature can predict phase behavior in active matter systems.

Findings

Quantitative agreement with equilibrium phase diagrams.

Observation of Gaussian displacement distributions.

Validation of fluctuation-dissipation relations.

Abstract

Thermal energy agitates all matter and its competition with ordering tendencies is one of the most fundamental organizing principles in the physical world. Thus, it is natural to enquire if an effective temperature could result when external energy input enhances agitation. Potentially this could extend the insights of statistical thermodynamics to nonequilibrium systems, but despite proposals that the effective temperature concept may apply to synthetic active matter, biological motors, granular materials and turbulent fluids, its predictive value remains unclear. Here, combining computer simulations and imaging experiments, we design a two-component system of driven Janus colloids such that collisions produced by external energy sources play the role of temperature, and in this system we demonstrate quantitative agreement with hallmarks of statistical thermodynamics for binary phase behavior: the archetypal phase diagram with equilibrium critical exponents, Gaussian displacement distributions, fluctuation-dissipation relations, and capillarity. These quantitative analogies to equilibrium expectations, observed in this decidedly nonequilibrium system, constitute an existence proof from which to compare future theories of nonequilibrium, but limitations of this concept are also highlighted.