Ultrasound-assisted detection and liquid systems

The uitrasonic properties of emuisions are usuaiiy frequency dependent, as are the effects of diffraction and transmission-refiection at muitiiayer boundaries. Thus, the frequency content of a US pulse used in an experiment is important. If a pulse contains a wide range of frequencies, then each frequency component will travel through a material at a different velocity and will be attenuated to a different extent, so only average values will be measured. This problem can be overcome by conducting experiments at a particular frequency or by using Fourier analysis to examine the frequency content of broad-band pulses. 

Determining the droplet size distribution of an emulsion by ultrasonic spectrometry involves two steps. First, the ultrasonic velocity and (or) attenuation coefficient of the emulsion is measured as a function of the frequency — preferably over as wide a range as possible. Second, the experimental measurements are compared with theoretical predictions of the ultrasonic properties of the emulsion, and the droplet size distribution providing the best fit between theory and experiment is determined. 

On the other hand, the fact that ultrasonic velocity is independent of droplet size In the low and high frequency limits allows droplet concentrations to be determined without prior knowledge of the droplet size distribution from ultrasonic velocity measurements. Whether measurements are to be made In the low- or high-frequency regime depends on the size of the droplets and the range of frequencies which can be measured using available ultrasonic equipment (typically 0.1-100 MHz). 

Ultrasonic techniques have also been used to study typical oll-ln-water emulsions as such and those undergoing either depletion flocculation or floe disruption. These studies are of industrial interest as flocculation of the droplets within an emulsion may often be the first stage in the deterioration of a product however, it can also be beneficial by Increasing the apparent viscosity of the product via the formation of a continuous network of droplets. 

Depletion flocculation has also been induced in oil-in-water emulsions by adding different concentrations of a non-adsorbing biopolymer (xanthan) to the aqueous phase. At low frequencies, the attenuation coefficient of the emulsions decreased with increasing 

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