Linear viscoelasticity of hard sphere colloidal crystals from resonance detected with dynamic light scattering

We present measurements of the high-frequency shear modulus and dynamic vis cosity for nonaqueous hard sphere colloidal crystals both in normal and mic rogravity environments. All experiments were performed on a multipurpose PH aSE instrument. For the rheological measurements, we detect the resonant re sponse to oscillatory forcing with a dynamic light scattering scheme-The re sonant response for colloidal crystals formed in normal and microgravity en vironments was similar, indicating that the bulk rheological properties are unaffected by differing crystal structure and crystallite size within the experimental error. Our high-frequency shear modulus seems reasonable, lyin g close to Frenkel and Ladd's predictions [Phys. Rev. Lett. 59, 1169 (1987) ] for the static modulus of hard sphere crystals. Our high-frequency dynami c viscosity, on the other hand, seems high, exceeding Shikata and Pearson [ J. Rheol. 38, 601 (1994)] and van der Werff et al.'s measurements [Phys. Re v. A 39, 795 (1989)] on the high-frequency dynamic viscosity for metastable fluids. The measurements are in the Linear regime for the shear modulus bu t may not be for the dynamic viscosity as Frith et al. [Powder Technol. 51, 27 (1987)] report that the dynamic viscosity passes through a maximum with strain amplitude. [S1063-651X(99)08708-5].