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

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 qualitative shape of the spectra is similar. These findings confirm recent theoretical predictions [A. C. Maggs, Phys. Rev. E 57, 2091 (1998)]. The discrepancy affects not only absolute values of G'(f) and G"(f): although microscopic and macroscopic plateau regime are found in the same frequency range, 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 microrheometry becomes large compared to the length of actin filaments.