It is well known that thermocapillary convection arises in liquid bridges when the isothermal support discs are at different temperatures. As the temperature difference increases the convective flow shows a transition from a steady ax-isymmetric to an oscillatory regime. Although a number of experiments have been performed and a considerable number of publications have appeared dur-ing the past ten years on oscillatory thermocapillary convection in liquid bridges, a definitive understanding of the physical mechanisms of the onset of instability and a coherent picture of the thermo-fluid-dynamic field has not yet emerged. On this subject the "Onset" experiment has been successfully performed during the D2 Spacelab mission using the Advanced Fluid Physics Module (AFPM). The experiment has investigated this transition for various bridge geometries and has provided quantitative measurements, which allow one to characterise the flow at the onset of the instability. The paper discusses the experimental ob-servations, compars them with the available theoretical models of the instability. This comparison provides a validation of the oscillation description in terms of hydrothermal waves and also a new interpretation of the experimental results obtained previously. Finally a new scaling law for the oscillation frequencies at the onset is presented which agrees with all available experimental data.
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