Thermocapillary flow instabilities in an annulus under microgravity — results of the experiment magia Original Research Article

We investigated thermocapillary flow in an annular gap with outer heater container of radius R1 = 40 mm and inner cooled cylinder of R2 = 20 mm and with an adjustable height h, 2.5 ≤ h ≤ 20 mm. The gap was filled flat up to the rim with the 0.65 cSt silicone oil hexamethyldisiloxane (Prandtl number Pr = 6.7). The temperature differences ΔT, 0 K ≤ ΔT ≤ 40 K between outer and inner wall generated thermocapillary flow in the free upper surface and various flow structures have been observed under microgravity. We identified hydrothermal waves for small h and more complicated oscillations for larger h. For small h and small ΔT the multiroll structure was visible via IR-images of the free surface: concentric steady convection rolls with the same sense of rotation, embedded into the main thermocapillary roll. We measured the critical Marangoni number Mac for the transition to time-dependent flow in the aspect ratio range A = h/(R1 − R2), 0.125 ≤ A ≤ 1, where it was virtually constant Mac ≈ 5 · 104. We report and discuss a steady temperature asymmetry, changing its direction from time to time, as recorded by the IR-camera. This symmetry breaking is most probably due to slowly changing residual acceleration in the satellite.