Examining the rotational diffusion of a microparticle suspended in a soft material opens up exciting new opportunities for locally probing the frequency-dependent linear viscoelastic shear modulus, $G^{*}(\omega )$. We study the one-dimensional rotational diffusion of a wax microdisk in an aqueous polymer entanglement network using light streak tracking. By measuring the disk’s time-dependent mean square angular displacement, $⟨\Delta {\theta }^2(t)⟩$, we predict the polymer solution’s $G^{*}(\omega )$ using a rotational generalized Stokes-Einstein relation. The good agreement of the predicted modulus with mechanical measurements confirms this new microrheological approach.

• Received 13 June 2002

DOI:https://doi.org/10.1103/PhysRevLett.90.018304