Thin oil films enable stable oil and water mixtures sans surfactant

Thin oil films absorbed onto the surface of water droplets lead to anomalously stable, surfactant-free oil and water mixtures, according to a new study. The findings demonstrate a mechanism for stabilizing water droplets in a water-oil emulsification without the need for a surfactant, which could have important technological applications, including the creation of very pure and controlled materials. Oil and water cannot form homogenous mixtures. Instead, when combined, droplets of one fluid will disperse inside the other, forming an emulsion. However, when two like droplets approach one another, the thin film separating them becomes unstable, causing them to coalesce into a single, larger droplet. Droplets will continue to merge until the fluids fully separate. The only known way to stabilize droplets and prevent them from spontaneously coalescing is by adding a surfactant to the mix. Claire Nannette and colleagues discovered an exception to this rule and report the spontaneous formation of stable water droplets within a polymeric oil. Nannette et al. show that when a thin layer of oil absorbs onto the surface of a water droplet, the oil at the interface becomes far more viscous than the surrounding bulk oil. This results in an attractive interaction between the two water droplets that drives a dramatic change in the dynamics of the oil molecules within the thin film separating the water droplets. This in turn leads to an extreme slowing of the formation of holes in the oil film separating two drops, which cause coalescence in other oil-water mixtures. According to the authors, water droplets in the polymeric oil can remain stable over timescales of several weeks, without the use of any surfactant or solvent.