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) approximately t(1/2) over two decades in time. It is flanked by a short-time regime with x(t) approximately t(3/4) and a viscous with x(t)approximately t. In the intermediate regime the creep compliance depends on the actin concentration c as c(-beta) with beta approximately 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) approximately t(1/2) and long-time regimes x(t) approximately t in which the compliance varies as c(-4/3), in agreement with experiment.