Effects of interfacial forces on the evaporation of a superheated water droplet in hot immiscible oil

The release of vapor bubbles from the surface of a superheated water droplet submerged in a hot immiscible oil was studied. Theoretical calculations indicate that the maximum bubble size and the residence time of the vapor bubble at the oil/water interface are controlled by the difference between the surface tensions at the oil/vapor and water/vapor interfaces. The interfacial forces act on the vapor bubble as a net force directed towards the oil phase. If the difference between the interfacial forces is substantial, the formation of strings if vapor bubbles can be predicted. When this difference is lowered by adding surfactant to the system, the bubble size and its residence time at the oil/water interface increase; this was confirmed by experiment. In the absence of a surfactant, there is a big difference between the respective surface tensions and, as a result, a string of tiny (0.2 mm) vapor bubbles is released from the oil/water interface for 1 to 2 min until the water droplet evaporates explosively, giving rise to the formation of a very large (40 mm) vapor bubble. In the presence of a surfactant (low difference between surface tensions) the medium size (5 mm) vapor bubble elongates the water droplet, finally causing timely fragmentation of the droplet.