THEORY OF ACOUSTIC EXCITATIONS IN COLLOIDAL SUSPENSIONS

Recent Brillouin scattering experiments have revealed the surprising existence of two acoustic modes in colloidal suspensions of solid spheres. Through an analytic Green's-function calculation and numerical simulations, we show that the higher-frequency mode is associated with the solid-sphere antiresonances, and the lower-frequency mode arises from the coupling of internal resonances between neighboring spheres. Our theory yields excellent agreement with the measured dispersion relations and offers and explanation for the observed frequency gaps in the excitation spectra.