J. Phys. IV France
Volume 03, Numéro C1, Mai 1993
Page(s) C1-183 - C1-196

J. Phys. IV France 03 (1993) C1-183-C1-196

DOI: 10.1051/jp4:1993116

Sound propagation in colloidal systems


Exxon Research and Engineering Co., Rt. 22 E, Annandale, NJ 08801, U.S.A.

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 (Φ-Φc)τ, where Φ is the volume fraction of droplets and Φc 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.

Nous etudions la propagation du son dans les systems colloidaux complexes. En combinant la diffusion Brillouin avec les techniques ultrasoniques, nous mesuront la dispersion de la propagation acoustique sur trois decades en frequence. La propagation acoustique est sensible a la compressiblite du millieu, et explore des proprietes nouvelles des systemes colloidaux. Nous etudions deux systemes colloidaux. Le premier est un systeme de micelles inverses ou la taille des micelles est plus petite que la longueur d'onde du son. En mesurant la dispersion de la celesite du son en fonction de la fraction volumique en micelles, nous identifiont une rigidite croissante du systeme a haute frequence. Le module croit comme (Φ-Φc)τ, ou Φ est la fraction volumique en micelles et [MATH]c est un fraction volumique critique, et Φc un exposant en accord avec une rigidite decrite par la theorie de la percolation. Le deuxieme systeme est compose de spheres dures dont le diametre est voisin de la longueur d'onde du son. Nous mesuront la courbes de dispersion des phonons a differentes fractions volumiques. Une nouvelle excitation acoustique est trouvee quand le longueur d'onde du son est comparable a la taille des spheres. Ce mode possede des propriete unusuelles et est attrubue a un excitation de surface qui peut se propager entre les spheres.

© EDP Sciences 1993

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