Standing waves of oscillatory thermocapillary convection in floating zones under microgravity observed in the experiment maus G141 Original Research Article

We report on the synchronous change of the temperature oscillation amplitude of oscillatory thermocapillary convection measured at three different positions in two floating zones with different length L under microgravity. The measurement was carried out with three thermocouples placed at different azimuthal positions. The amplitude changes occurred in the region of two times the critical Marangoni number and are of different magnitude for the three thermocouples. We interpret this by the occurrence of two hydrothermal waves of opposite azimuthal travelling direction originating from a source. This can be intepreted as a standing wave which is azimuthally fixed. The source can move to a different azimuthal position and gives rise to the above described observation. This state of superimposed waves may at the same time release the system from the restriction m · λϕ = 2πR (m = even number, λϕ = wavelength in azimuthal direction, R = zone radius) which is not easily fulfilled for every aspect ratio L · R−1. Thus it not be an ideal standing wave. There is evidence that the waveform of standing waves is the preferred waveform under microgravity and that this waveform is changed to travelling waves by the action of gravity. But because travelling waves are easily disturbed and can change to travelling waves this conclusion needs further experimental verification.