Department of Physics & DEAS, Harvard University, Cambridge, Massachusetts 02138, USA.
We measure the viscoelasticity of entangled F-actin over length scales between 1 and 100 microm 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 rad/sec; this is probed with one-particle microrheology. Longitudinal fluctuations of the filaments increase the elastic modulus between 0.1 and 30 rad/sec at length scales up to the filament persistence length; this is probed by two-particle microrheology.