Microscopic theory of step-rate start-up shear rheology: Insights into the overshoot, shear-thinning, and structural relaxation time evolution using recovery rheology.

Recovery rheology separates the deformation strain into recoverable and unrecoverable components and has provided new insights into the yielding of soft matter systems across various rheological protocols. An interesting finding is that the fluid-like response associated with the gradually increasing unrecoverable strain ultimately leads to yielding. We extend the microscopic Elastically Collective Nonlinear Langevin Equation theory of activated dynamics under nonequilibrium conditions within a microrheological generalized Maxwell model framework to analyze step-rate start-up shear response of dense glass forming hard-sphere fluids and colloidal suspensions in a recovery rheology framework. New physical inter-connections between macroscopic experimental variables and microscopic properties are predicted. (1) The recoverable strain in steady state is directly related to the steady-state Peclet number and shear-thinning behavior. (2) The transient stress overshoot amplitude varies non-monotonically with packing fraction and is quantitatively linked to the steady-state recoverable strain. (3) The predicted enormous reduction of the stress and structural relaxation time under active deformation is inversely related to the unrecoverable strain-rate.