Osmotic force-controlled microrheometry of entangled actin networks

In studying a magnetic bead's creep response to force pulses in an entangled actin network we have found a novel regime where the bead motion obeys a power law x(t)∼t1/2 over two decades in time. It is flanked by a short-time regime with x(t)∼t3/4 and a viscous with x(t)∼t. In the intermediate regime the creep compliance depends on the actin concentration c as c-β with β 1.1±0.3. We explain this behavior in terms of osmotic restoring force generated by the piling up of filaments in front of the moving bead. A model based on this concept predicts intermediate x(t)∼t1/2 and long-time regimes x(t)∼t in which the compliance varies as c-4/3, in agreement with experiment.