We measure the viscoelasticity of entangled F-actin over length scales between $1$ and $100\mu \mathrm{m}$ using one- and two-particle microrheology, and directly identify two distinct microscopic contributions to the elasticity. Filament entanglements lead to a frequency-independent elastic modulus over an extended frequency range of $0.01–30\mathrm{r}\mathrm{a}\mathrm{d}/\sec $; this is probed with one-particle microrheology. Longitudinal fluctuations of the filaments increase the elastic modulus between 0.1 and $30\mathrm{r}\mathrm{a}\mathrm{d}/\sec $ at length scales up to the filament persistence length; this is probed by two-particle microrheology.

• Received 28 March 2003

DOI:https://doi.org/10.1103/PhysRevLett.91.158302