Viscoelastic response of actin networks

The viscoelastic response of actin networks is length- and time-scale dependent, encoding information on intrinsic dynamic correlations and mesoscopic structure. Over sufficiently large distances the network responds as a continuous medium, characterized by frequency-dependent viscoelastic moduli. But how large should the distance be for this asymptotic bulk limit to hold?

We report the observation of a large-distance intermediate response in an experimental system of F-actin networks. The tools of 1-point and 2-point microrheology were used to characterize the local and distance-dependent responses of the actin networks, respectively. The 2-point response at intermediate distances, arising from the effect of mass displacement rather than momentum diffusion, is enhanced by the much softer local microenvironment of the tracers compared to the bulk properties of the gel. Consequently, the cross-over to the bulk behavior is pushed to surprisingly large distances, much larger than the mesh size, , of the actin gel.

By developing a new analysis scheme for microrheology experiments, combining both 1-point and 2-point measurements, we were able to characterize the intermediate response, which in turn allows extracting the material’s structural properties. We use this newfound understanding to extract structural information of active in-vitro reconstituted cytoskeleton networks, in which such analysis can be done in a controlled fashion.