Sessile volatile drop evaporation under microgravity

Abstract

The evaporation of sessile drops of various volatile and non-volatile liquids, and their internal flow patterns with or without instabilities have been the subject of many investigations. The current experiment is a preparatory one for a space experiment planned to be installed in the European Drawer Rack 2 (EDR-2) of the International Space Station (ISS), to investigate drop evaporation in weightlessness. In this work, we concentrate on preliminary experimental results for the evaporation of hydrofluoroether (HFE-7100) sessile drops in a sounding rocket that has been performed in the frame of the MASER-14 Sounding Rocket Campaign, providing the science team with the opportunity to test the module and perform the experiment in microgravity for six consecutive minutes. The focus is on the evaporation rate, experimentally observed thermo-capillary instabilities, and the de-pinning process. The experimental results provide evidence for the relationship between thermo-capillary instabilities and the measured critical height of the sessile drop interface. There is also evidence of the effects of microgravity and Earth conditions on the sessile drop evaporation rate, and the shape of the sessile drop interface and its influence on the de-pinning process.

Numerical model for sessile drop evaporation on heated substrate under microgravity

“…As expected evaporation under microgravity is lower than terrestrial gravity [33,34] and the deviation continue to increase with the drop liquid volatility and substrate temperature as observed in the case of Ethanol and HFE7100. Also, the comparison of the average of the evaporation rate in microgravity (numerical) and terrestrial gravity (experiments) for water at substrate temperature 𝑇 = 313 K, evaporation rate under microgravity is 8.85% lower than the under terrestrial gravity condition but difference increase further up to 18.14% at 𝑇 = 343 K [35,36].…”

“…As some diffusion models (mostly one-way coupled [8,10,11,15,38,39,40]) also considered evaporation of sessile drop under saturated vapour pressure with gravity. However under same conditions the evaporation rate under gravity is higher compared to microgravity [34,37]. In gravity, natural convection influence (reduces) the vapour pressure, so vapour pressure should be not be considered equal to saturated vapour pressure.…”

“…During the evaporation process, sessile drop evaporation rate is mainly controlled by the vapour diffusion in the surrounding atmosphere. The energy transfer from substrate to the interface of sessile drop under microgravity conditions is considered mainly due to Marangoni convection [34]. Marangoni convection occurs inside the sessile drop as an effect of surface tension gradient due to its temperature dependency.…”

Sessile drop evaporation in a gap -- crossover between diffusion-limited and phase transition-limited regime

“…The good agreement with the Stokes results for narrow gaps for small or moderate contact angles indicates that the developed thin-film model can be used as a valid tool for the study of shallow sessile drops of volatile liquids in a variety of contexts across all dynamical regimes. The model can easily be adapted for many closely related systems, e.g., incorporating chemically or topographically heterogeneous substrates, resulting in contact line pinning and depinning (van Der Heijden et al 2018), adding gravity or other body forces (Kumar et al 2020), or considering the importance of the evaporation regime in dip coating (Bindini et al 2017;Dey et al 2016). As the presented thin-film description is of gradient dynamics form, necessary changes can often be incorporated via adapting the underlying free energy functional that here only incorporates wetting, interface and bulk energies of the liquid and vapour entropy [cf.…”

Sessile drops in weightlessness: an ideal playground for challenging Young’s equation

“…A new macroscopic mechanical model was derived to compute the shape of weightless sessile drops in static equilibrium. Our initial motivation was to understand more clearly the repeatability issues encountered while creating sessile drops in weightlessness by injecting liquid through a small hole in a substrate 21 , 22 . The derived model is based on the classical Young–Laplace equation, Eq.…”