Influence of Microgravity and Marangoni Flow on the Vapor Distribution Surrounding an Evaporating Sessile Droplet*

In microgravity, thermal Marangoni flow in an evaporating sessile droplet of a pure volatile liquid is expected to be dominant enough to affect advection in the gas and hence to alter the vapor distribution around the droplet and the evaporation rates. In order to test this hypothesis, the vapor cloud surrounding a hydrofluoroether (HFE-7100) sessile droplet has been visualized using a Mach-Zehnder interferometer. The microgravity conditions were produced in a series of parabolic flight manoeuvres organized by ESA. During the tests, the phase-wrapped images obtained from vapor interferometry show that the vapor cloud is not only influenced by the Marangoni flow but also by the inherent g-jitters arising from the flight itself. However, the impact of g-jitters is observed to be significant only during the later stages of the microgravity phase. The vapor cloud, initially hemispherical, rises and forms a plume-like structure. Although there is interference from the g-jitters, this plume is mostly a consequence of a fully developed Marangoni convection inside the droplet. The evaporation rates were measured by tracking the droplet volume evolution. On the contrary, the experiments conducted on ground showed a vapor cloud flattened by gravity with different evaporation rates.