Spreading and hole formation in natural oil films on aqueous solutions

Abstract

The dynamics of a natural oil film on a free surface of the water has been studied experimentally. We examine the process of spreading and breakup of an oil film on water as well as on dilute aqueous solutions of Octanoic acid. Oil from Cocos Nucifera (Coconut) is used as the test fluid. A drop of this test fluid is introduced on to the free surface of the substrate fluid. The spatio-temporal evolution of the oil film is imaged and the features are investigated. The oil drop evolves into a film and spreads spontaneously because of the positive spreading coefficient. At a certain critical film thickness, holes, which are voids in the oil film, begin to nucleate. When the hole area fraction approaches the random packing limit, these holes begin to coalesce resulting in a coarser structure of the oil film. Coalescence occurs when the liquid bridge separating a pair of holes is ruptured due to Rayleigh–Plateau instability. This process is completely spontaneous since it is driven by gradients in the interfacial energy landscape. The oil film was observed to evolve through three phases viz., (i) spreading phase, (ii) hole nucleation and growth phase and (iii) hole coalescence phase. Quantitatively, the oil film diameter, number of holes in the oil film, diameter of each hole as well as the total triple line length has been measured. We also investigate the effect of Octanoic acid concentration in the substrate fluid on this dynamics. We show that increasing Octanoic acid concentration in the substrate fluid slows down the dynamics of film break up. This is possibly due to increased hydrophobic/hydrophilic interaction forces between the substrate fluid and the oil film.