Abstract:

We measure the dispersion of the longitudinal sound waves in a suspension of solid spheres using Brillouin scattering. We fmd two distinct propagating longitudinal modes when the wavelength of the sound becomes comparable to the sphere diameter. The higher-frequency mode has a velocity intermediate between those of the pure solid and pure liquid phases, and its velocity increases with increasing solid volume fraction. The dispersion curve of this mode has distinct gaps, and the group velocity goes to zero near these gaps. We interpret this mode as a compressional ''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 spheres, and occur at frequencies where the scattering from a single, isolated 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 less than those in either the pure solid or the pure fluid. We interpret this mode as a surface acoustic excitation, which propagates between adjacent spheres by means of the exponentially decaying portion of the excitation in the fluid at the surface of the spheres. A summary of a theoretical treatment is also presented.

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