Abstract
The onset of thermal convection in a double layer of two superimposed immiscible fluids heated from below is investigated. The linearized equations of the problem are analysed in a much wider region of the parameter space than has been studied before. It is shown that the onset of steady convection in the two layers may occur in the form of either viscously or thermally coupled motions. In addition to the oscillatory interfacial instability, which depends on a non-vanishing distortion of the interface, there exists another oscillatory instability which corresponds to a cyclic variation between viscous and thermal coupling. Conditions for the onset of this instability are outlined and its connections with the other modes of the system are demonstrated in bifurcation diagrams. In the experiments the shadowgraph method is used for the visualization of the onset of convection and for the measurement of its wavelength. Changeovers between viscous and thermal coupling can be identified, but the experimental realization of an oscillatory onset has been elusive so far.
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