Abstract
The flocculation of emulsion droplets has been monitored using low intensity ultrasound. A series of 20 wt.% n-hexadecane oil-in-water emulsions (average droplet diameter d 32 = 0.55 μm) containing different concentrations of free surfactant in the aqueous phase (0–12 wt.% polyoxyethylene sorbitan monolaurate) was prepared. Enhanced droplet creaming was observed at free surfactant concentrations greater than 7 wt.%, which was attributed to depletion flocculation caused by the exclusion of surfactant micelles from the narrow region separating droplets that are close together. The ultrasonic velocity and attenuation coefficient of the emulsions were measured at 20.0°C from 1–4 MHz. The ultrasonic measurements indicated that scattering of ultrasound by the emulsions decreased dramatically when droplets flocculated. Ultrasound therefore offers a sensitive tool for investigating the structure of concentrated and optically opaque emulsions.
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