Bubbles and Drops will move in a liquid matrix despite of the absence of buoyancy in weightlessness, if the surface (separating area of the two phases) exhibits differences in surface tension. These can be caused by temperature differences at the bubble or drop periphery, since the surface tension depends on the temperature. In this situation the surface of the bubble or drop drives a flow, which is directed towards increasing surface tension. This phenomenon is termed thermocapillary convection and was confirmed and systematically investigated in space shuttle experiments under reduced gravity in a cooperation between the TU Bergakademie Freiberg (Germany), the Department of Chemical Engineering of the Clarkson University, Potsdam, New York, and the NASA Lewis Research Center.

An important motivation for this research is the through space flight generated possibility to produce novel materials with an improved quality. The reason for this is the absence of gravity dependent buoyant convection, which is believed to be the cause for a limited product quality in earth bound production processes. The investigations showed, that bubbles or drops in a liquid with temperature gradient move faster, the higher the applied temperature gradient and the larger the bubble or drop diameter is. Therefore, it is basically possible to move bubbles and drops in a liquid despite of the absence of gravity.

The first film sequence shows the injection of a bubble (air) and the following migra-tion through the liquid matrix (silicone oil) with temperature gradient under microgravity within the space shuttle. Bubble diameter: 8.2 mm; temperature gradient: 0.33 K/mm. The second sequence displays this experiment visualized by a laser interferometer. The fringe pattern gives indications on the temperature field of the flow.

The interested reader can find further details in the following publications:

  • Thermocapillary migration of bubbles and drops at moderate to large Marangoni number and moderate Reynolds number in reduced gravity.
    P. H. Hadland, R. Balasubramanian, G. Wozniak, R. S. Subramanian.
    Experiments in Fluids 26 (1999) 240 – 248
  • Temperature fields in a liquid due to the thermocapillary motion of bubbles and drops. G. Wozniak, R. Balasubramaniam, P. H. Hadland, R. S. Subramanian.
    Experiments in Fluids 31 (2001) 84 – 89