On the instability of water-in-oil emulsion drop lamella and rim moving over a heated surface in the film boiling regime. (March 2023) - Piskunov, Maxim
Highlights: Linked lamella and rim instability of emulsion drop after impact on a solid surface is examined. Instability occurs in the film boiling regime. Mechanism is proposed where fingering causes the lamella rupture near attachment to the rim. Emulsion drop rim is destabilized due to dispersed water droplets perturbing lamella surface. Role of emulsion concentration and a convection in lamella and rim instability is revealed. Abstract: The experimental study deals with the nature of interrelated unstable equilibrium of lamella and rim of an emulsion drop based on n -dodecane and bidistilled water after its collision with a solid surface heated to 350–400 °C at Weber numbers We =160–310 and in the presence of a stable boundary vapor layer. The relationship between the delay time of the onset of emulsion lamella destabilization relative to maximum drop spreading time and the ratio of the characteristics of diffusive and convective heat transfer at unstable equilibrium of the liquid is revealed. The combined effect of We and the emulsion concentration on the critical Rayleigh and Marangoni numbers characterizing the lamella perforation is demonstrated. A mechanism is proposed where the fingering causes the lamella rupture near attachment to the rim. Liquid wettability is partially restored and the liquid periodically contacts the surface, resulting in the vapor bubbles formation and growth, as well as their collapse, which ensures the cascade of the lamella destruction and the liquid bridges formation. The proposed physical model develops ideas about the complexity and interconnectedness of the destabilization mechanisms of the emulsion drop rim and lamella, including not only rim instabilities, but also the long-wave Marangoni instability taking into account the emulsion concentration and, in some cases, Rayleigh-Benard instability.