ABSORPTION AND VELOCITY DISPERSION DUE TO CRYSTALLIZATION AND MELTING OF EMULSION DROPLETS

The influence of droplet crystallization and melting on the ultrasonic properties of oil-in-water emulsions has been investigated. The ultrasonic velocity and attenuation were measured in a series of 3 wt% n-hexadecane-in-water emulsions as a function of frequency (0.3-4 MHz), droplet diameter (0.4 and 1 mu m) and temperature (0-25 degrees C). The emulsified n-hexadecane crystallized at about 5 degrees C due to supercooling effects and melted at about 18 degrees C. As solid and liquid n-hexadecane have significantly different ultrasonic properties, an appreciable change in the velocity and attenuation is observed during the phase transition. This behaviour is modified significantly in systems where the emulsion droplets are partially crystalline because the temperature fluctuations associated with the ultrasonic wave can perturb the phase equilibria solid reversible arrow liquid causing excess attenuation and velocity dispersion. The magnitude of this effect depends on the ultrasonic frequency and the average droplet size.