Investigation of the film formation mechanism of oil-in-water (O/W) emulsions

Abstract

To reveal the film formation mechanism of oil-in-water (O/W) emulsions, the thicknesses of nanofilms of an aqueous paraffin oil emulsion, stabilized with the nonionic Tween 80 and Span 80 surfactants, were measured in confinement between two solid surfaces, by the use of the relative optical interference intensity (ROII) approach. Such films’ thicknesses were found to be sensitive to the rolling speed. In contrast to a single-phase oil lubricant, which as an elastohydrodynamic film had a thickness that always increased with speed when there was a sufficient supply of new material, the film formation of oil-in-water emulsions normally has a hill-like appearance (the film undergoes a collapse to a relatively low thickness at a critical speed after the evident rise in thickness with increasing rolling speed). The critical speeds for film formation of O/W emulsions with various emulsifiers and oil concentrations were focused on to gain an insight into the film formation mechanism of oil-in-water emulsions. Droplets can be observed to concentrate and break up before the contact at a low speed, which induces an oily pool. The oily pool seemed to act as the provider of the lubrication of the contact during the rolling process. The re-emulsification effect was employed to explain the collapse of the film thickness as the speed exceeded the critical value. A theoretical model was proposed to describe the re-emulsification effect, which established a relationship between the critical speed and the concentrations of either the oil or the emulsifiers.