Dancing Vapor Clouds Above Evaporating Sessile Droplets: Results of 78th ESA Parabolic Flight Campaign

As a series of microgravity tests before a forthcoming space experiment aboard the International Space Station, evaporation of a sessile droplet of a pure liquid (here a refrigerant HFE-7100) into a quiescent nitrogen atmosphere has been studied in a parabolic flight campaign. The ambient conditions were those of the aircraft (19.5°C and 850 mbar, somewhat below the normal conditions), the contact line was pinned at a circular microgroove of a radius of 2 mm in the substrate, and droplet volumes ranged between about 6 µL and 10 µL (contact angles from 45° to 70°). Despite some anomalies in the injection system, also encountered in previous experiments, we eventually estimated evaporation rates by observing droplet volume changes with a side-view camera. A noticeable influence of residual gravity fluctuations in the plane ('g-jitter', up to 10-2g here) was revealed, especially for negative-g periods, rendering the present experiment not exactly 0g. Even if the roughness of volume data differentiation with time did not permit to follow the evaporation rate fluctuations in response to the g-jitter and only averaged values were actually measured, this gap was bridged by vapor interferometry and numerical simulations. In particular, 'dancing' vapor clouds were thereby revealed, which turned out to be in a good agreement with simulations. The role of an intense thermal Marangoni circulation inside the droplet under g-jitter conditions was also explored in simulations.