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Everything flows… but how ? A unified state diagram for the yielding transition of soft glassy materials
“Everything flows…” could have said the Philosopher opening the fridge for breakfast. Indeed, soft glassy materials such as foams, creams, butter and mayonnaise can flow if we shake, push or spread them, while they stay firm and retain their shape like solids when we stop applying an external force. Such mechanically induced fluidization is called yielding and is key in countless applications. Yet, the mechanism through which this flow initiates and the very nature of this transition have been elusive so far. Is yielding a transition between two distinct physical states, akin to the melting of a solid ice cube upon raising temperature, or the liquid-vapor transition of a boiling liquid ? Or is it a continuous transition between two qualitatively similar dynamical states ?
In this talk, I will discuss experiments in which we investigate the microscopic signature of mechanical yielding in various soft materials, by simultaneously measuring their mechanical response and microscopic dynamics. I will show that these dynamics change qualitatively upon yielding, and that this change can be described by an Ising-like model formally identical to that describing thermodynamic phase changes in real gases. I will build upon this analogy to propose a unified state diagram for the yielding transition of soft glassy materials, allowing to quantitatively compare samples with different microscopic and mechanical properties, and to reconcile previously contrasting observations on the nature of yielding.