a b s t r a c tThe hydrodynamic stability of a thermocapillary flow in a finite-size liquid bridge, with a non-deformable interface, heated from above and surrounded by a passive gas is examined by means of three-dimensional direct numerical simulations. Convective flow in 1 cSt silicone oil with Pr ¼ 18 is driven by combined effects of buoyancy and thermocapillary forces. Effects of the interfacial heat exchange on hydrothermal stability of the bulk flow are evaluated for various external thermal conditions in the gas phase. Cooling the interface can significantly shift the bifurcation point where the thermocapillary flow becomes oscillatory. If the Biot number Bi, which is a measure of the rate of the heat exchange, is not large, the effect of the interfacial heat flux can be either stabilizing or destabilizing, depending on both the temperature profile in the gas and the height of the liquid bridge. For a high value of heat loss rate, stabilization occurs regardless of the temperature distribution in the ambient gas. Various flow regimes are identified, and detailed stability maps are made for the flow under the considered ambient thermal conditions. It was found that the critical azimuthal mode of the flow changes according to the surrounding conditions in the gas phase. Observations have revealed three distinct azimuthal modes: m ¼ 0 (critical for a long liquid bridge at small Biot numbers), m ¼ 1 and m ¼ 2. Any mode change of the flow is accompanied by an abrupt change of the frequency of temperature oscillations.

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A series of experiments has been performed under earth's gravity to study formation of particle accumulation structures (PAS) in a supercritical flow driven by the combined effects of buoyancy and thermocapillary forces. The test flow was created in a non-isothermal cylindrical column (liquid bridge) made of n-decane and heated from above. The objective of the experiment was to answer two major questions: (1) how strong is the influence of the shape of the interface on the process of formation of PAS;(2) what temperature of the ambient air fits better for PAS to occur. Considering these questions, we developed a method based on changing both the volume of the liquid bridge and temperature at the external walls of the experimental chamber to set and to keep constant the shape of the interface and the temperature inside the setup, respectively. The experimental observations are presented in the form of diagrams in the parameters' space showing ranges of the PAS formation. The findings show that a liquid bridge with an interface as close to the straight cylindrical as possible and surrounded by air at low temperature is the best terrain for D. Melnikov ( ) · V. Shevtsova Université Libre de Bruxelles (ULB), MRC, CP-165/62, PAS formation. The results of the chaos analysis of the recorded temperature time series and their correlation with the obtained diagrams allow for showing that accumulation of particles in coherent structures is possible only in a periodic oscillatory flow characterized by a small value of the translation error not exceeding 0.01. It is demonstrated that presence of either any spectral noise or of several modes with incommensurate frequencies makes formation of a PAS impossible.