Dynamics of Dewetting on a Liquid Substrate

Dewetting, the phenomenon of a thin film rupturing to form beads due to interplay between Van Der Waals forces and intermolecular forces, is a ubiquitous phenomenon occurring naturally in our surroundings. A thin metal film dewetting on a solid substrate is a classic example of an industrially applicable dewetting phenomenon where the dewetted thin beads act like an etch mask. However, there is not much literature about dewetting phenomenon on a liquid substrate i.e. dewetting of a thin liquid film on top of another liquid with no influence of a solid substrate locally. This type of system is prevalent in nature, especially in oil spills where in the oil drops rise above and gently impact the water-air interface. Very little is known about the oil droplet behavior upon impacting the interface and just before spreading. In this research, the behavior of the droplet immediately after the thin film dewets was investigated, wherein it was found that upon thin film rupture, the oil drop does not start spreading immediately. Instead, it undergoes deformation and stretching, due to the propagation of capillary waves through the phase boundary, causing a pinch-off mechanism resulting in a production of a smaller daughter droplet. For large daughter droplet formation, this process repeats itself, resulting in a cascade of self-similar events. This has significant implications in aquatic ecosystems where the naturally acting surfactants trap the smaller daughter droplets produced and inhibit it from reaching the interface. Using a high-speed imaging camera, the behavior of the oil drop at the bulk liquid-air interface was captured and the size of the daughter drop produced was analyzed. Various concentrations of water-glycerol concentrations were used to change the bulk fluid parameters. The results for different oil drops and oil drop sizes were obtained and a regime map demarcating region of jetting and region of no jetting was established. The size of the daughter droplet produced was found to be directly dependent on the bulk fluid parameters. Various parameters involved in droplet deformation preceding to pinching-off were also studied and a threshold Ohnesorge Number was determined for each parameter after which no daughter droplet is generated. A relationship between size of the daughter droplet and each parameter studied was also established. Eventually, it is determined that the size of the daughter droplet formed is dependent on the strength of the capillary waves meeting at the apex.