Viscoelasticity of entangled actin networks studied by long-pulse magnetic bead microrheometry.

We studied the viscoelastic response of entangled actin networks using embedded microbeads driven by force pulses with amplitudes in the range from 3 to 120 pN and durations up to 60 s. We distinguished three regimes in the time dependence of the compliance J(t) of the network. These were characterized by specific power laws J(t) approximately t(alpha)(i) (i=1, 2, 3). In the short-time regime (i=1), we observed the exponent alpha1 approximately 0.75. In the long-time regime (i=3), we find that alpha3 approximately 1. For the intermediate-time interval (i=2), we observed a novel dynamic regime: for all actin concentrations and all applied forces, it was characterized by the exponent alpha3 approximately 0.5. In both regimes i=2 and i=3, the compliance depended upon the actin concentration c, such as J approximately c(-gamma)(i) with gamma2 approximately 1.1 and gamma 3 approximately 1.4. Using these results, we calculated the shear modulus in the frequency domain and found that the intermediate-time regime in the t domain corresponds to its plateau behavior.