SOUND-PROPAGATION IN SUSPENSIONS OF SOLID SPHERES

We measure the dispersion of the longitudinal sound waves in a suspens ion of solid spheres using Brillouin scattering. We fmd two distinct p ropagating longitudinal modes when the wavelength of the sound becomes comparable to the sphere diameter. The higher-frequency mode has a ve locity intermediate between those of the pure solid and pure liquid ph ases, and its velocity increases with increasing solid volume fraction . The dispersion curve of this mode has distinct gaps, and the group v elocity goes to zero near these gaps. We interpret this mode as a comp ressional ''citation which propagates through both the liquid and the solid, as expected for a composite medium. The gaps in the dispersion curve result from the very large scattering of the excitation by the s pheres, and occur at frequencies where the scattering from a single, i solated sphere is predicted to be a maximim due to a resonance in the sphere. By contrast, the lower-frequency mode has a velocity that is l ess than those in either the pure solid or the pure fluid. We interpre t this mode as a surface acoustic excitation, which propagates between adjacent spheres by means of the exponentially decaying portion of th e excitation in the fluid at the surface of the spheres. A summary of a theoretical treatment is also presented.