We show that complex fluids such as actin networks respond differently to deformations at intermediate length scales than they do at large distances as a bulk material. This intermediate response regime is characterized by a 1/r³ decay with distance. When characterizing passive entangled F-actin networks, we observed this intermediate response at surprisingly large distances of 2-6 µm, which are comparable to the size of a cell, and are over ten times larger than the mesh size of the actin network. We generalize the framework of microrheology to include and characterize this 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.