Transfer of Oil between Emulsion Droplets

A contrast matching technique was used to determine the exchange of oils between emulsion droplets having different refractive indexes. Emulsions of tetradecane and 1-bromo tetradecane in water were made separately in a high-pressure homogenizer, then mixed and equilibrated at rest. It was found that droplets exchanged oil molecules through the continuous phase, in a process similar to Ostwald ripening. Emulsions of hexadecane and 1-bromo hexadecane were also mixed; at rest, no exchange of oil took place. These mixtures were subsequently recirculated in the high-pressure homogenizer. Exchange of oil occurred as a result of droplet coalescence in the homogenizer. Two regimes were found, "surfactant-poor" and "surfactant-rich." In the "surfactant-poor" regime, recoalescence took place at all values of the pressure used in the homogenizer. In the "surfactant-rich" regime, recoalescence took place only if the pressure was at least equal to that used originally to make the emulsion. These results demonstrate that the size of the emulsion droplets made in a high-pressure homogenizer results from a succession of fragmentation and recoalescence processes. Possible mechanisms preventing recoalescence are discussed.