Irreversibility in disordered microfluidic droplet ensembles governed by three-body scattering

Viscous (Stokes) flow is symmetric under time reversal, but the presence of solid particles or droplets breaks this symmetry. Identifying elementary microscopic processes that break time reversal symmetry in these systems is an open problem. Here we use a dilute disordered dispersion of microfluidic droplets to distinguish three-body collisional scattering as the elementary irreversible process in the otherwise reversible viscous flow. In this process, three droplets interacting by long-range hydrodynamic dipoles approach and collide to form a cluster that quickly breaks into a pair and single that move apart. The pair creation is accompanied by a local increase in spatial order, which is measured by a reduction of the three-body configurational entropy. Our results put forth an elementary mechanism for reversibility breaking in particle carrying fluids and highlight the importance of three-body motion as a source of complexity in many-body systems with long-range interactions.