ACOUSTIC PROPERTIES OF SOLID-LIQUID MIXTURES AND THE LIMITS OF ULTRASOUND DIAGNOSTICS .1. EXPERIMENTS

Ultrasound as a technique for interrogating two-phase mixtures has the advantages of being nonintrusive, it has a very high frequency respon se, and is able to penetrate typically opaque highly concentrated mixt ures. There exists, however, an inherent compromise in the choice of t he frequency of the ultrasound between maximizing spatial resolution a nd ensuring adequate beam penetration. To this end, the propagation of ultrasound in solid-liquid mixtures has been investigated experimenta lly for a range of frequencies and concentrations of the dispersed pha se. The measured attenuation has been shown to depend roughly linearly on frequency for 0.1 < kr < 0. 75 (where the wavenumber k = 2pi/lambd a, and lambda and r are the wavelength and particle radius, respective ly), and quadratically for kr > 0. 75. As a function of solids concent ration, the attenuation displays a maximum at a solids fraction of abo ut 30 percent for the present system of silica beads in water. This ro bust and reproducible result contradicts models of attenuation that re ly on linear superposition of single particle effects. The intensity f ield produced by a circular disk transducer in a two phase medium at k r approximately 1 shows excellent agreement with the Rayleigh integral with a modified wavenumber and attenuation parameter, and it allows f or the prediction of the transducer beam geometry in two phase mixture s for a wide range of frequencies and solids fractions. The limitation s of ultrasonic wave propagation as a nonintrusive diagnostic techniqu e, in terms of spatial resolution, have been discussed. Acknowledging these limitations, an ultrasonic instrument for determining the veloci ty of moving particles at or near maximum packing was built. Prelimina ry results from this prototypical ultrasonic Doppler velocimeter show good agreement with observations of the settling velocity of silica be ads at high concentrations.