Microscopic basis for recovery rheology and the nonequilibrium structure, yielding, and flow of dense particle suspensions

The recent introduction of recovery rheology has provided qualitatively new physical insights into the yielding and flow of soft matter systems across diverse mechanically driven nonequilibrium protocols by separating the deformation strain into recoverable and unrecoverable components. A striking finding is that the fluidlike response associated with the gradually increasing unrecoverable strain ultimately leads to the continuous yielding transition from a solid to a liquid. We build on the force and particle level Elastically Collective Nonlinear Langevin equation theory of activated dynamics within a nonequilibrium microrheological framework to formulate a general statistical mechanical foundation of step-rate start-up shear response that relates recovery rheology to microscopic structure, relaxation, and elasticity. Quantitative applications to metastable hard and soft sphere colloidal suspensions reveal testable predictions and interconnections between macroscopic and microscopic properties: (1) the steady-state recoverable strain is directly related to the steady-state shear thinning; (2) the transient stress overshoot amplitude varies nonmonotonically with packing fraction and is quantitatively linked to the steady-state recoverable strain; (3) the acquired unrecoverable strain dictates the stress overshoot strain; and (4) the predicted enormous reduction of the structural relaxation time under deformation is inversely related to the unrecoverable strain rate.