Microrheometry underestimates the values of the viscoelastic moduli in measurements on F-actin solutions compared to macrorheometry

We present a systematic comparison of microrheological and macrorheological measurements of the viscoelastic storage and loss moduli, G'(f) and G "(f) , respectively, of solutions of the semiflexible biopolymer F-actin. Using magnetic tweezers microrheometry and rotating disk macrorheometry, we show that microscopic values for G'(f) and G "(f) are significantly smaller than macroscopic results over the frequency range f = 0.004-4 Hz, whereas the q ualitative shape of the spectra is similar. These findings confirm recent t heoretical predictions [A. C. Maggs, Phys. Rev. E 57, 2091 (1998)]. The dis crepancy affects not only absolute values of G'(f) and G "(f): although mic roscopic and macroscopic plateau regime are found in the same frequency ran ge, the two methods yield different values for the entanglement time which determines the high-frequency end of the plateau. By investigating F-actin solutions of different mean filament lengths, we show that microscopic and macroscopic G'(f) and G "(f) converge, if the probe particle used in micror heometry becomes large compared to the length of actin filaments.