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) ∼ t αi (i=1, 2, 3). In the short-time regime (i=1), we observed the exponent α1≈0.75. In the long-time regime (i=3), we find that α3≈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 α3≈0.5. In both regimes i=2 and i=3, the compliance depended upon the actin concentration c, such as J∼ c- γi with γ2≈1.1 and γ3≈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.