A Microscopic View of Oil Slick Break-up and Emulsion Formation in Breaking Waves

The maximum stable drop size for dispersions arising from oil slicks and water in oil emulsions are shown to be controlled by Raleigh–Taylor instability or the prevalent local shear stress in breaking waves, which ever is more restrictive. Data from five experimental studies, including oils and oil in water emulsions with densities less than that of water and viscosities ranging from 0.001 to 1000 Pa s were fitted quantitatively by the model. Low viscosity oils tend to break into drops with a sauter equivalent diameter less than 0.005 m, while oils with a high viscosity and a density approaching that of water remain as large slicks. Water in oil emulsions can be formed by dispersion inversion i.e., water drops can become trapped among oil drops coalescing at the water–air surface and by water drop entrainment i.e., water entrained by gas bubbles escaping from the collapsing vortices of breaking waves, can be released in the form of fine drops as the bubbles pass through large oil slicks. Both mechanisms are shown to be active over a broad range of oil physical properties. Viscous oils with a low oil–water interfacial tension yield the most stable water in oil emulsions in breaking waves.