We perform local or microrheological measurements on microtubule solutions, as well as composite networks. The viscoelastic properties of microtubules as reported from two-point microrheology agree with the macroscopic measurement at high frequencies, but appear to show a discrepancy at low frequencies, at time scales on the order of a second. A composite of filamentous actin (F-actin) and microtubules has viscoelastic behavior between that of F-actin and pure microtubules. We further show that the Poisson ratio of the composite, measured by the length-scale dependent two-point microrheology, is robustly smaller than that of the F-actin network at time scales $\tau >1\mathrm{s}$, suggesting that a local compressibility is conferred by the addition of microtubules to the F-actin network.

• Received 1 September 2008

DOI:https://doi.org/10.1103/PhysRevLett.102.188303