Response of a Complex Fluid at Intermediate Distances

The viscoelastic response of complex fluids is length- and time-scale dependent, encoding information on intrinsic dynamic correlations and mesoscopic structure. We derive the subdominant response of such fluids at intermediate distances and show that it governs their dynamics over surprisingly large length scales. Generalizing the framework of microrheology to include this response, we experimentally confirm the theory, thereby measuring the dynamic correlation length of F-actin networks, as well as their bulk and local viscoelastic properties.