RHEOLOGY AND MICRORHEOLOGY OF SEMIFLEXIBLE POLYMER-SOLUTIONS - ACTIN FILAMENT NETWORKS

We report a systematic study of the linear rheology of solutions of mo del 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 mu M and aft er full polymerization, the elastic and loss moduli still increase wit h time as G'(t) proportional to t(0.25+/-0.02) and G ''(t) proportiona l to t(0.15+/-0.03), when measured at 1 rad/s, during network formatio n and reach a plateau after 12 h. At equilibrium, the linear small-fre quency elastic modulus has a small magnitude, G(p)' = 14 +/- 3 dynes/c m(2). The magnitude of G(p)', depends weakly on concentration as G(p)' (c) proportional to c(1.2+/-0.2), with an exponent much smaller than f or flexible polymers. At large concentrations, F-actin network becomes a liquid crystal and G', is independent of concentration Using the la rge bandwidth of DWS, we show that; the high-frequency viscoelastic mo dulus of F-actin solutions varies with the shear frequency as \G(omeg a)\ proportional to omega(0.78+/-0.10) for both the isotropic phase an d liquid crystalline phase. These results are in good agreement with a recent model of semiflexible polymer solutions (the ''curvature-stres s'' model) and reflect the finite rigidity of F-actin.