Title: Softening and Yielding of Soft Glassy Materials,
Abstract: Solids deform and fluids flow, but soft glassy materials, such as emulsions,
foams, suspensions, and pastes, exhibit an intricate mix of solid and
liquid-like behavior. While much progress has been made to understand their
elastic (small strain) and flow (infinite strain) properties, such
understanding is lacking for the softening and yielding phenomena that connect
these asymptotic regimes. Here we present a comprehensive framework for
softening and yielding of soft glassy materials, based on extensive numerical
simulations of oscillatory rheological tests, and show that two distinct
scenarios unfold depending on the material's packing density. For dense
systems, there is a single, pressure-independent strain where the elastic
modulus drops and the particle motion becomes diffusive. In contrast, for
weakly jammed systems, a two-step process arises: at an intermediate softening
strain, the elastic and loss moduli both drop down and then reach a new plateau
value, whereas the particle motion becomes diffusive at the distinctly larger
yield strain. We show that softening is associated with an extensive number of
microscopic contact changes leading to a non-analytic rheological signature.
Moreover, the scaling of the softening strain with pressure suggest the
existence of a novel pressure scale above which softening and yielding
coincide, and we verify the existence of this crossover scale numerically. Our
findings thus evidence the existence of two distinct classes of soft glassy
materials -- jamming dominated and dense -- and show how these can be
distinguished by their rheological fingerprint.