Photographic Observations of the Transient Contact Behavior of Oil-in-Water Emulsion Droplets Impinging on a Hot Substrate

Oil-in-water (O/W) emulsions are widely used as the working fluids in various metalworking processes. In this study, the transient liquid/solid contact behavior of O/W emulsion droplets impinging on a hot sapphire prism was experimentally investigated in order to gain insight on spray jet impingement in industrial applications. Three-directional flash photography method (with three digital cameras and four flashlights) was used to obtain the time evolutions of the droplet shape and liquid/solid contact behavior. Water, base oil, and O/W emulsions (oil concentration: 5, 15 mass%) were chosen as the test liquids. The experiments were conducted under a droplet impact velocity of 1.0 m/s and the pre-impact diameter of the droplets was similar to 2.3 mm. The temperature of the substrate was varied from 150 degrees C to 400 degrees C. The results showed that there was less direct liquid/solid contact at higher substrate temperatures because the boiling of water was more intense. There was no direct liquid/solid contact at a substrate temperature of 400 degrees C. In addition, the direct liquid/solid contact areas were smaller for the O/W emulsion with higher oil concentration because of the vapor trapped between the solid surface and oil-rich liquid with high viscosity. Furthermore, it was found that the oil adhesion on the test substrate rarely occurred except at lower substrate temperatures.