Abstract
Droplet flocculation in oil-in-water emulsions has been monitored by ultrasonic attenuation spectroscopy, a technique which is sensitive to changes in the spatial distribution of the droplets within an emulsion. The ultrasonic attenuation spectra (1-150 MHz) of a series of 5 wt% corn oil-in-water emulsions (m) were recorded at 25C. Depletion flocculation was induced in the emulsions by adding various concentrations (7-100 mM) of sodium dodecyl sulphate to the aqueous phase. At low frequencies, the attenuation coefficient of the emulsions decreased with flocculation due to overlap of the thermal waves generated by the droplets. At high frequencies, the attenuation coefficient increased with flocculation due to an increase in scattering by the flocs. The results could be explained using an effective medium theory recently developed to account for the influence of droplet flocculation on the ultrasonic properties of emulsions. The ultrasonic technique was also used to monitor the breakdown of flocs under shear flow. The dependence of the ultrasonic properties of emulsions on flocculation means that ultrasonic attenuation spectroscopy can be used to study interactions among droplets in concentrated emulsions.
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