It is common knowledge that the sharp distinctions between the solutal and thermal convection are caused by large diffusion time of the former [1, 2]. These differences become more severe in the presence of the liquid ? liquid interface. A deep and rather slow gravitational convection supplies a surfactant to the interface causing thereby intensive outbursts of the Marangoni convection. As experiments show, the coexistence of the solutal gravitational convection and capillary convection usually leads to the onset of the oscillatory mode. In this paper we investigate several systems, in which the development of the oscillator mode of the solutal convection was observed experimentally. The other distinguishing feature of the solutal convection in the liquid system with the interface is attributed to the fact that diffusion of the surfactant molecules to the interface and formation of the surface temperature perturbations are governed by different mechanisms. In the surface phase generated at the free surface of a fluid, concentration of the surfactant is defined by a competition of adsorption and desorption processes. Therefore, the mechanism responsible for the onset of thermocapillary convection differs from that of the solutal Marangoni convection, which begins, as experiments show, after the formation of a certain finite gradient of surfactant concentration at the surface [3]. The value of the critical surfactant gradient depends on the degree of preliminary fluid purification. However, in our experiments we failed to reduce it to zero. Therefore in the numerical simulations the surface phase is assumed to exhibit Bingham properties ? the elements of the surface phase are set into motion as soon as the tangential stresses exceed some limiting value.
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