Rotational and translational phonon modes in glasses composed of ellipsoidal particles

The effects of particle shape on the vibrational properties of colloidal glasses are studied experimentally. "Ellipsoidal glasses" are created by stretching polystyrene spheres to different aspect ratios and then suspending the resulting ellipsoidal particles in water at a high packing fraction. By measuring displacement correlations between particles, we extract vibrational properties of the corresponding "shadow" ellipsoidal glass with the same geometric configuration and interactions as the "source" suspension but without damping. Low-frequency modes in glasses composed of ellipsoidal particles with major-to-minor axis aspect ratios of ~1.1 are observed to have predominantly rotational character. In contrast, low-frequency modes in glasses of ellipsoidal particles with larger aspect ratios (~3.0) exhibit a mixed rotational and translational character. All glass samples were characterized by a distribution of particles with different aspect ratios. Interestingly, even within the same sample it was found that small-aspect-ratio particles participate relatively more in rotational modes, while large-aspect-ratio particles tend to participate relatively more in translational modes.