Hydrodynamic instabilities under microgravity in a differentially heated long liquid bridge with aspect ratio near the Rayleigh-limit: Experimental results

A liquid bridge of 15.0 mm length L and 3.0 mm radius r (aspect ratio A = L/r = 5) from 2 cSt silicone oil (Pr = 28) was established under microgravity during the flight of the sounding rocket MAXUS-4. Four different temperature differences ΔT = 7, 9, 10, 12 K have been applied between the ends for sufficient time to reach steady state thermocapillary flow conditions. The aim of the experiment - to observe the onset of hydrothermal waves and to measure their features, like the waves phase speed and the angle between the wave vector and the applied temperature gradient - was reached. Moreover, we can report about the occurrence of a further hydrodynamic structure or instability occurring at the same time in this experiment, namely convection cells of the type of the Bénard-Marangoni instability, drifting in the surface flow from the hot towards the cold side. The latter instability has not been observed in the liquid bridges with A = 1 investigated normally, because of geometric restrictions. The Bénard-Marangoni instability is due to the cooling of the free liquid surface in our experiment.