Phonons in colloidal systems

Rich phonon spectra were observed experimentally by Brillouin spectroscopy in liquid, glassy, and crystalline state of colloidal systems of low and high elastic constant contrast. The nature of these phonons was elucidated by theoretical calculations of the single sphere scattering cross section, the energy density distribution, the light scattering intensity, and determination of the band structure by the multiple scattering method. Besides the ordinary acoustic phonon, localized optic-like modes, mixed modes, and Bragg induced modes were identified. Their relation to the physical state of a colloidal suspension sensitively depends on the micromechanical mismatch between particle and surrounding medium and the coherence of the crystalline structure. Polycrystalline colloidal suspensions show distinct acoustic excitations in the high wave vector–low frequency region.