Rotational diffusion in concentrated colloidal dispersions of hard spheres

We have performed depolarized dynamic light scattering measurements on suspensions of colloidal spherical particles made of a fluorinated polymer. Electrostatic interactions are screened by adding salt, so that the particles behave as hard spheres. By suspending the particles in an index-matching solvent (18% urea in water) we have been able to investigate a wide range of particle volume fractions Φ from the dilute suspension up to 55%. The partially crystalline internal structure of fluorinated polymer colloids gives rise to a significant depolarized component in the scattered light field. By studying the temporal fluctuations of the depolarized component we can evaluate the short-time self-translational and rotational diffusion coefficients of the particles over the whole colloidal fluid phase and within the hard-sphere colloidal crystal. We also analyze the full shape of the rotational correlation function, which deviates from an exponential behavior in concentrated suspensions. Starting from the generalized Smoluchowski equation, we calculate the Φ dependence of the rotational diffusion coefficient up to the Φ² term, and we discuss the validity of the approximation that decouples translational from rotational motion. We find a good agreement between theoretical and experimental results.