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

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.