We analyzed particle motions in colloidal crystals by digital video microscopy, and obtained the dispersion relations of amplitude and relaxation time for normal relaxation modes of particle dynamics. Because the observed crystals were soft, the soft mode which had larger amplitude and longer relaxation time appeared in -M. Such soft mode, however, could not be reproduced at all by a simple overdamped Langevin theory in which the hydrodynamic interaction and long-range interaction were not incorporated. Therefore we attempted to introduce the influence of flow into the equation of motion by using the Oseen tensor. The results of the modified model were in quantitative agreement with the experimental results. We also found that it was essential to consider the negative force constant between 3rd-nearest neighbor particles for explaining the soft mode.
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