Memory of shear flow in jammed suspensions

Shearing jammed suspensions of soft particles results in heterogeneous and correlated dynamics leading to stress heterogeneities. To assess the role of such complex dynamics in encoding memory of shear, we perform shear cessation simulations and experiments at various shear rates. Consistent with previous work we find that the preshear-rates set the time scales of stress-relaxation at early times, which is correlated with a ballistic-like motion, reminiscent of the non-affine short-time particle dynamics observed during shear. An investigation of the correlations of the short-time non-affine displacement fluctuations observed just before and after shear cessation reveals remarkable similarities in the dynamical correlations observed in both regimes. This indicates that the spatially heterogeneous dynamics during shear imprints stress inhomogeneities that in turn drive spatially heterogeneous dynamics upon shear cessation. Beyond the short-time behavior, we find that the long-time relaxation towards a residual stress is connected to the evolution of local stiffness. Our findings suggest that steady-state shear is a process through which a memory of the imposed shear rate and of the distribution of local yield stresses is built into the material, a memory that can be then effectively read out in a shear cessation test.