Experimental study of the onset of the 3D oscillatory thermocapillary convection around a single air or vapor bubble.

In a first section, Marangoni convection induced by a temperature gradient is studied around a single air bubble. The bubble is introduced under a downward facing heating element into a liquid layer heated from upper and cooled from below (silicone oils Pr = 16.7 and Pr = 228 or FC-72, Pr = 12.3). In this configuration stationary thermocapillary convection can be followed by the oscillatory convection. Firstly the 3D oscillatory thermocapillary convection is presented for a silicone oil (Pr = 16.7). Then, the critical threshold between the stationary state and the first oscillatory mode is determined. The influence of the liquid kind on this threshold is presented for one layer height. Moreover, other parameters (horizontal wall temperature, layer height) are tested for one liquid. At least, we consider heat transfer induced by this convection, and we discuss the results versus the convection state. The second section of this paper concerns preliminary results on Marangoni convection with boiling around a single vapor bubble growing under a downward facing heating element into a subcooled liquid (FC-72). The oscillatory thermocapillary convection is observed. It occurs during the growth from a critical bubble radius. The influence of the subcooling level on its occurrence threshold is shown.