Bilayer Rayleigh-Marangoni convection: transitions in flow structures at the interface

Abstract

The fluid physics of buoyancy-driven (Rayleigh) and interfacial tension-driven (Marangoni) convection is examined for two superimposed layers of fluids. This con­vection occurs on account of temperature gradients that are imposed perpendicular to the fluid-fluid interface. Interfacial deflections, small as they may be, play an important part in identifying the mechanism that governs the flow, and calculations have been made that indicate whether hot or cold fluid flows towards or away from a crest or a trough. As a result, four possible flow structures or modes at the interface have been identified. Two heating styles, heating from below and above, are com­pared and the behaviour of the fluid physics as a function of total fluid depths, depth ratios and gravity levels is explained. Changes in modes result because of changes in these parameters. We have given plausible physically based arguments that predict the sequential change in modes as these parameters are changed and have veri­fied our conjectures with calculations. Flow mechanisms in the case of a solidifying lower phase have also been studied, as this has an application to liquid-encapsulated crystal growth. Where convection is deemed detrimental to crystal homogeneity, we conclude that the liquid-encapsulated method of crystal growth is best conducted under Earths gravity.