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Critical viscoelastic response in jammed solids

We determine the linear viscoelastic response of jammed packings of athermal repulsive viscous spheres, a model for emulsions, wet foams, and soft colloidal suspensions. We numerically measure the complex shear modulus, a fundamental characterization of the response, and demonstrate that low frequency response displays dynamic critical scaling near unjamming. Viscoelastic shear response is governed by the relaxational eigenmodes of a packing. We use scaling arguments to explain the distribution of eigenrates, which develops a divergence at unjamming. We then derive the critical exponents characterizing response, including a vanishing shear modulus, diverging viscosity, and critical shear thinning regime. Finally, we demonstrate that macroscopic rheology is sensitive to details of the local viscous force law. By varying the ratio of normal and tangential damping coefficients, we identify and explain a qualitative difference between systems with strong and weak damping of sliding motion. When sliding is weakly damped there is no diverging time scale, no diverging viscosity, and no critical shear thinning regime.