Viscoelastic 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 study the length scale above which bulk viscoelasticity sets in, and the material response that precedes it at shorter distances. We show that the crossover between these two regimes may appear at a surprisingly large distance. We generalize the framework of microrheology to include both regimes and apply it to F-actin networks, thereby extracting their dynamic correlation length from their bulk and local viscoelastic properties.