Impact of initial conditions and gas dynamics on the evaporation of a sessile droplet in microgravity and on-ground explained by a numerical model

The wide interest in the evaporation dynamics of sessile droplets has led to many studies aiming at understanding specific phenomena and overall behaviour. The effect of controlling the initial conditions, or the lack thereof, has not been studied extensively. One of the main issues are the conditions under which the droplet is injected on the substrate. This problem does not only raise questions on an experimental basis but also for numerical predictions. In the context of preparatory work for future space experiments in a sounding rocket and on board the International Space Station, initial conditions are difficult to control and their complete understanding lacks, whereas simulations may shed light on the subject. This work develops a comprehensive axisymmetric transient model that describes the injection of a sessile droplet evaporating into a gas phase as well as the evaporation process of that droplet. The model is validated against experimental data in both Earth and microgravity conditions. The amount of injected volume has been shown to increase the evaporation rate, with a weaker influence at microgravity with respect to Earth conditions. Comparing our model to the case where no injection history is present reveals that neglecting the latter has no significant influence on the evaporation dynamics nor on the simulated velocity and temperature profiles. This suggests that in most cases it is not necessary to include the injection period, so that uncertainties involving the injection are not an obstacle to model the evaporation of sessile droplets. Moreover, it also appears that thermal supply by the injected fluid plays a large role during injection in suppressing Marangoni convection, whilst the latter becomes the main mechanism determining the flow patterns after the injection. Finally, it appears that for high-molar-mass vapours the gas dynamics play an important role in the evaporation process. In particular, for correct numerical simulations, it is necessary to consider concentration-dependent gas properties, and especially concentration-dependent gas density, these effects being most often neglected in other approaches, which results in underestimated evaporation rates.