Visualization of water transport in W1/O/W2 emulsions

Water transport between two aqueous phases and through an intervening oil phase under osmotic pressure was observed and quantified visually using capillary video-microscopy. Under certain conditions, a new mechanism was observed directly, according to which the pure-water phase emulsifies spontaneously and the resulting emulsified droplets migrate to the saline aqueous phase. Another finding was the importance of the thickness of the oil phase, O, that separates two aqueous bodies, W1 and W2, in that it determines which transport mechanism of water between W1 and W2 will be predominant. Specifically, in a W1/O/W2 emulsion globule where W1 represents the internal pure-water droplets and W2 the suspending saline-water medium, when W1 and W2 were at visual contact, water transport occurred mainly through the hydrated surfactant mechanism. In the case of a visible minimum distance of separation between W1 and W2, measuring from a few to over 100 μm, the water transport rate was found to be significantly lower than the rate at visual contact and water migration occurred via spontaneously emulsified droplets and reverse micelles. In all cases, the transport rate was independent of the size of the water droplets and the oil globule, and in the case of no visual contact, it was also independent of the minimum separation distance between W1 and W2. This result implies that, under the experimental conditions used, the water transport rate in W1/O/W2 emulsions is controlled by interfacial processes, rather than being diffusion controlled as has been suggested by previous work.