SOUND-PROPAGATION IN COLLOIDAL SYSTEMS

We study the propagation of sound in complex colloidal systems. By combining Brillouin scattering with ultrasonic techniques, we measure the dispersion in the acoustic propagation over three decades in frequency. Acoustic propagation is sensitive to the bulk compressibility of the medium, and probes new structural and dynamic properties of the colloidal system. We study two colloidal systems. The first is a system of inverted micelles or microemulsions, where the droplet size is significantly smaller than the wavelength of the sound. By measuring the dispersion of the sound velocity as a function of droplet volume fraction, we identify an increased rigidity of the system at high frequencies. The increase in the modulus scales as (phi - phi(c))tau, where phi is the volume fraction of droplets and phi(c) is a critical volume fraction. This is consistent with rigidity percolation. The second system we study consists of a suspension of hard sphere colloids whose diameter is comparable to the wavelength of sound. We measure the dispersion curve for the phonons in this system at different volume fractions of spheres. A new acoustic excitation is found when the wavelength of the sound is comparable to the sphere diameter. This acoustic excitation possesses unusual properties and is attributed to a surface excitation that can propagate coherently between adjacent spheres.