Abstract Systems governed by time reversible equations of motion often give rise to
irreversible behaviour,,. The transition from reversible to irreversible behaviour is
fundamental to statistical physics, but has not been observed experimentally in many-body
systems. The flow of a newtonian fluid at low Reynolds number can be reversible: for
example, if the fluid between concentric cylinders is sheared by boundary motion that is
subsequently reversed, then all fluid elements return to their starting positions. Similarly …

Slowly sheared suspensions of solid particles are governed by time-reversible equations of
motion. Here we report a precise experimental test showing that time-reversibility fails for
slowly sheared suspensions. We study a dense suspension of PMMA particles (index and
density matched to the fluid), at low Reynolds number in a Couette cell using oscillatory
strain. We find that there is a concentration-dependent threshold strain amplitude beyond
which particles do not return to their starting configurations after one or more cycles. Instead …