Pine, Chaikin and collaborators [Nature (2005), Nature Physics (2008)]have recently studied the transition between reversible andirreversiblebehavior in a simple experiment. The experiment consists of observingand studying the trajectories of a large number of particles suspendedina fluid sheared between two coaxial cylinders. These experimentsillustrate a novel way in which a driven many-particle system canself-organize (termed“random organization” by these authors). They also illuminateseveral problems which lie at the foundations of statisticalmechanics, such as theconnection between irreversible behaviour at the macroscopic scale andmicroscopic reversibility at the microscale.I will first describe these experiments and then present a simplemodel for the underlying physics. Predicting universal features of thereversible-irreversibletransition seen in these experiments then becomes possible. Thesepredictions are in reasonable agreement with what is seen in theexperiments. I will also brieflydescribe the connection to recent ideas concerning the “jamming”transition in granular materials, ideas that may have broaderimplications for theories of the glasstransition.
