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Nonequilibrium Transitions Govern the Flow of Glasses
The understanding of glasses remains one of the grand challenges in condensed matter : glasses have liquid-like structure, but exhibit solid-like properties. These materials inspire us to re-think the notion of flow and rigidity of solids. Recent theory and experiments suggest novel nonequilibrium transitions to underlie the flow of glasses. Such nonequilibrium transitions would connect a wide range of amorphous materials - from nano-pillars to bulk metallic glasses to granulates and earth quakes - in a universal theory of deformation. Experimentally, new “soft glasses”, whose constituent particles can be imaged and tracked accurately in three dimensions such as colloidal suspensions, foams and emulsions, have evolved as important model systems to obtain direct insight into the physics of glasses. In this talk, I will present recent evidences for non-equilibrium transitions using flowing and aging soft glasses. Depending on the way of driving, continuous and discontinuous non-equilibrium transitions are observed. The experiments show that these transitions, which occur in the dynamics, are also accompanied by very weak, but resolvable structural changes. These new kinds of phase transitions occur at the verge from rigidity to flow, demarcating a profound change in the particle dynamics. Using hard-sphere systems to directly measure the free energy from particle configurations, we exploit the relationship between strain build-up and relaxation, (structural) free energy changes and dynamics at the onset of flow.