Nonequilibrium glass-melting mechanism for two step yielding in step-rate shear and post-cessation re-equilibration dynamics in Brownian attractive glasses and dense gels.

Abstract

Microscopic nonequilibrium statistical mechanical theory is employed to construct a unified framework for the prototypical soft matter system of dense Brownian colloidal suspensions with short-range attractive interactions under continuous step-rate shear deformation and the subsequent re-equilibration dynamics after shear cessation. The novel approach connects quiescent glass-melting, double yielding, two-step post-cessation relaxation, physical bond formation, and aging. The time-dependent cohesive energy, deeply related to nonequilibrium structure on the nanometer scale, quantifies physical bonding strength in a many body manner. It specifically connects the deformation-driven response to known non-monotonic quiescent activated dynamics and elastic modulus re-entrancy phenomena as attraction strength is varied. The non-monotonic evolution with strain of the elastic modulus from an attraction-dominated bonding regime to one controlled by repulsive cage constraints is the origin of double-yielding. It is mechanistically related to significant cohesive energy surviving beyond the first yield point at low strains, thereby explaining the experimentally observed extremely large second yield point strain. Upon shear cessation, a two-step stress and structural relaxation process is predicted corresponding to a fast, initial strain-rate-dependent repulsive/entropic cage recovery process that retains deformation memory, followed by an extremely slow relaxation associated with the reformation of short-range attractive bonds, which results in the emergence of long-lived residual stresses and deformation memory on experimental timescales. The rich dependence of these dynamics on attraction strength and range, packing fraction, and pre-shear rate is systematically explored, and provides new insights for controlling material rheological response and properties with strong implications for printing-based fabrication and soft matter ink design.