Non-equilibrium thermodynamics in driven macroscopic self-assembly

Abstract

Equilibrium statistical mechanics provides a robust framework for characterizing phase transitions in systems whose microsopic dynamics are time-reversible. Efforts to develop and validate theoretical frameworks for time-irreversible, non-equilibrium systems are constrained by experimental data that capture only partial measurements of the system dynamics. We herein overcome this limitation using a tunable macroscopic platform for non-equilibrium physics: millimetric spheres bound by capillary attractions at the fluid interface and driven out of equilibrium by a field of supercritical Faraday waves. The external driving induces correlated fluctuations in the particle trajectories, which in turn excite structural rearrangements between distinct metastable cluster topologies. By tracking all microstate transitions experimentally, we directly measure a non-zero entropy production rate reflecting broken detailed balance and quantifying the system's departure from equilibrium. The measured stochastic dynamics are in quantitative agreement with a many-body active Ornstein-Uhlenbeck model, thus establishing a bridge to a wider class of athermal, self-propelled systems at the microscale. These results invite parallel studies of non-equilibrium self-assembly kinetics using active colloids or passive particles immersed in bacterial baths whose dynamics and irreversibility are likewise governed by correlated active forces and tunable inter-particle interactions.