Rheology of F-actin solutions determined from thermally driven tracer motion

We report measurements of the frequency-dependent complex shear modulus of semidilute F-actin solutions based on optical observations of the thermally-excited motion of monodisperse tracer microspheres. Because the tracer spheres cause incident laser light to be strongly scattered, we determine their average motion using diffusing wave spectroscopy (DWS). From the measured mean square displacement, we extract the retardation spectrum of the actin solution using a regularized fit based on a discretized model involving a linear superposition of harmonically bound brownian particles. At an actin concentration of C = 1.2 mg/ml and for microspheres of radius a = 0.8 μm, we find that the complex modulus exhibits a dominant low frequency plateau modulus and a high frequency rise with the loss modulus dominating above a crossover frequency. Over a limited range of frequencies well above the crossover frequency, the magnitude of the high frequency storage modulus, G'(ω), is consistent with the power law scaling ω3/4. The observed gradual crossover appears to be at odds with previous theoretical predicitons, but it corresponds to a simple structure of the retardation spectrum.