Rotational dynamics of rigid rods. A dynamic light-scattering study

Abstract

We report measurements of the homodyne light-scattering correlation function, ∣S(k, t)∣², for the rod-like virus M-13 over the concentration range 0.20–2.0 mg cm^–3. In dilute solution the correlation function shows the usual double-exponential behaviour expected for a rod executing essentially free translational and rotational diffusion. At higher concentrations the initial decay is less rapid and there is clear evidence of a slowly decaying long-time “tail”. Experimentally we find the initial slope in the semi-dilute regime to be about half that found in dilute solution and at long times ∣S(k,t)∣²∝t^{–1.1 ± 0.2}. The above results are in good agreement with the recent theoretical predictions of Doi and Edwards (DE). However, at the wavevectors probed in the present study the DE equation for S(k, t) leads to rather unphysical estimates for both the rotational and translational diffusion coefficients. A possible rationale for this behaviour is briefly discussed.