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 sph eres cause incident laser light to be strongly scattered, we determine thei r average motion using diffusing wave spectroscopy. From the measured mean square displacement, we extract the retardation spectrum of the actin solut ion using st regularized fit based on a discretized model involving a linea r superposition of harmonically bound Brownian particles. At an actin conce ntration of C = 1.2 mg/ml and for microspheres of radius a = 0.8 mu m, we f ind that the complex modulus exhibits a dominant low frequency plateau modu lus and a high frequency rise with the loss modulus dominating above a cros sover frequency. Over a limited range of frequencies well above the crossov er frequency, the magnitude of the high frequency storage modulus G'(omega) is consistent with the power law scaling omega(3/4). The observed gradual crossover appears to be at odds with previous theoretical predictions, but it corresponds to a simple structure of the retardation spectrum. (C) 2000 The Society of Rheology. [S0148-6055(00)00904-4].