Rheology and microrheology of semiflexible polymer solutions: Actin filament networks

We report a systematic study of the linear rheology of solutions of model semiflexible polymers, actin filaments (F-actin), using mechanical rheometry, diffusing wave spectroscopy (DWS), and video-based single-particle tracking microrheology. For pure actin at c = 24 μM and after full polymerization, the elastic and loss moduli still increase with time as G′(t) ∞ t^0.25±0.02 and G″(t) ∞ t^0.15±0.03, when measured at 1 rad/s, during network formation and reach a plateau after 12 h. At equilibrium, the linear small-frequency elastic modulus has a small magnitude, G′_p = 14 ± 3 dynes/cm². The magnitude of G′_p depends weakly on concentration as G′_p(c) ∞ c^1.2±0.2, with an exponent much smaller than for flexible polymers. At large concentrations, F-actin network becomes a liquid crystal and G′_p is independent of concentration. Using the large bandwidth of DWS, we show that the high-frequency viscoelastic modulus of F-actin solutions varies with the shear frequency as |G*(ω)| ∞ ω^0.78±0.10 for both the isotropic phase and liquid crystalline phase. These results are in good agreement with a recent model of semiflexible polymer solutions (the "curvature-stress" model) and reflect the finite rigidity of F-actin.