Discontinuous Jamming Transitions in Soft Materials

Many systems in nature exhibit transitions between fluid-like states and solidlike states, or “jamming transitions”. Such transitions can be induced by changing thermodynamic quantities, such as temperature or pressure (equilibrium transitions) or by changing applied stress (non-equilibrium transitions). There is a strong theoretical foundation for understanding equilibrium phase transitions that involve solidification, or jamming. Other jamming transitions, such as the glass transition, are less wellunderstood. The jamming phase diagram has been proposed to unify the description of equilibrium phase transitions, the glass transitions, and other non-equilibrium jamming transitions. As with equilibrium phase transitions, which can either be first order (discontinuous in a relevant order parameter) or second order (continuous), one would expect that generalized jamming transitions can be continuous or discontinuous. In studies of flow in complex fluids, there is a wide range of evidence for discontinuous transitions, mostly in the context of shear localization, or shear banding. In this paper, I review the experimental evidence for discontinuous transitions. I focus on systems in which there is a discontinuity in the rate of strain between two, coexisting states: one in which the material is flowing and the other in which it is solid-like. Discontinuous Jamming Transitions in Soft Materials 2