Oscillatory magnetic bead rheometer for complex fluid microrheometry

The dynamic range of the microrheometry based on the analysis of the enforced motion of colloidal force probes by video microscopy has been extended to 40 Hz. For that purpose a novel rapid image processing system has been developed enabling the real-time data acquisition and analysis thus improving the time resolution of particle tracking to 6 ms. A second advancement consists of the development of a correction procedure accounting for two effects: First, for the smearing out of the diffraction image of the beads due to the finite data acquisition time and, second, for systematic phase shifts of magnetic bead deflection with respect to the force due to the finite response time of the superparamagnetic beads in the direction of the gradient of the magnetic field. The new method has been applied to re-study the dynamic scaling law of the frequency dependence of the viscoelastic impedance of entangled actin solutions in the frequency regime determined by the conformational dynamics and entropic tension of single filaments. The frequency dependence of the storage and loss modulus obeys the scaling laws $G^{\prime }{\mathrm{(\omega )\propto G}}^{″}{\mathrm{(\omega )\propto \omega }}^{\alpha }$ with $\text{α=0.83(8)}$ which is only slightly higher than the theoretical prediction $\text{(α=0.75).}$