Introduction and the Principle of Ultrasonic Scattering

The two intrinsic losses represent the attenuation that occurs in each of the bulk phases. The attenuation through the emulsion will be related to some volume-averaged combination of the two attenuation coefficients. The other losses are due to scattering. Reflectional losses occur when there are differences between the acoustic impedance of the two phases. The reflected sound is not absorbed by the system, but is scattered out of the path of the forward travelling wave and thus lost to the receiver. 

Thermal losses (above those contributing to the intrinsic attenuation of each phase) occur when there is a difference between the specific heat capacities of the two phases. Heat is generated and dissipated by the cyclic pressure fluctuations within the sound wave. When the specific heat capacities of the two phases are 

A loss mechanism similar in form to thermal expansion losses arises when there is a difference between the compressibility of the drop and the supervening fluid. Once more the drop expands and contracts relative to the fluid, re-radiating energy, but this time as a direct consequence of the fluctuating pressure field. 

Additionally, this theory assumes that the scattering droplets are randomly positioned within the continuous phase and makes no allowance for droplet-droplet interactions. It is thus reasonable to expect deviations between theory and experiment for systems where the particles are flocculated. Recent work by McClements demonstrated that the attenuation and velocity of ultrasonic waves changed significantly when a sufficient concentration of surfactant micelles was added to an emulsion. Visual observations of the emulsion suggested that the ultrasonic changes were caused by the micelles acting as depletion flocculants. 

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