LOCAL MEASUREMENTS OF VISCOELASTIC MODULI OF ENTANGLED ACTIN NETWORKSUSING AN OSCILLATING MAGNETIC BEAD MICRO-RHEOMETER

A magnetically driven bead micro-rheometer for local quantitative meas urements of the viscoelastic moduli in soft macromolecular networks su ch as an entangled F-actin solution is described. The viscoelastic res ponse of paramagnetic latex beads to external magnetic forces is analy zed by optical particle tracking and fast image processing. Several mo des of operation are possible, including analysis of bead motion after pulse-like or oscillatory excitations, or after application of a cons tant force. The frequency dependencies of the storage modulus, G'(omeg a), and the loss modulus, G''(omega), were measured for frequencies fr om 10(-1) Hz to 5 Hz. For low actin concentrations (mesh sizes xi > 0. 1 mu m) we found that both G'(omega) and G''(omega) scale with omega(1 /2). This scaling law and the absolute values of G' and G'' agree with conventional theological measurements, demonstrating that the magneti c bead micro-rheometer allows quantitative measurements of the viscoel astic moduli. Local variations of the viscoelastic moduli (and thus of the network density and mesh size) can be probed in several ways: 1) by measurement of G' and G'' at different sites within the network; 2) by the simultaneous analysis of several embedded beads; and 3) by eva luation of the bead trajectories over macroscopic distances. The latte r mode yields absolute values and local fluctuations of the apparent v iscosity eta(x) of the network.