High-frequency dynamics and microrheology of macromolecular solutions probed by diffusing wave spectroscopy: the case of concentrated solutions of F-actin

The linear viscoelastic properties of macromolecular solutions and networks are traditionally investigated by probing their mechanical response upon applied shear. However, a small-amplitude strain may be insufficient to reach the torque resolution of a mechanical rheometer, especially for weak hydrogels. Such materials may require large deformations, which can orient and even rupture the constitutive polymers. This paper reviews a recently-developed approach to probe the mechanical properties of macromolecular networks and solutions non-invasively in the linear regime, and over an extended frequency range. This approach consists in measuring the thermally-induced displacement of microspheres imbedded within the solution, from which equilibrium, linear viscoelastic moduli can be extracted. The displacement of the probing microspheres is measured by diffusing wave spectroscopy (DWS). The extended bandwidth of DWS provides for new insight into the short-time and short-length scales motion of individual polymers in solution and offers a more stringent test of models of polymer dynamics. Here, we illustrate the use of DWS on a semi-dilute solution of model semi-flexible polymers: a concentrated actin-filament network.