Effect of an external Electric Field on Bubbles and Sessile Drops in Microgravity Environment: Interfacial Dynamics and Heat Transfer

This Thesis focuses on vapor bubbles and evaporating sessile drops, and on the effect of an external electric field on such processes. These apparently simple systems encompass a huge number of physical phenomena at different scales, which make them still not completely understood nowadays. A bubble and a drop have much in common, and in particular the presence of a liquid-vapor interface and of a triple region. Despite that, the processes in which are involved - boiling and evaporation respectively - are distinct. The use of an external electric field by means of an electrode has potential beneficial effects from the heat transfer point of view for both boiling as well as evaporation. In reduced gravity environments, it is especially accentuated, as the electric forces arising at the interfaces may constitute an action similar to the gravity force. Concerning boiling, the advantages of the electric force consist mainly of induced bubble detachment and vapor removal from the heated surface, but they can include also electroconvection and heat transfer enhancement at the triple line and the contact region. Regarding drop evaporation, electric forces modify the wettability and can influence the evaporation rate. These hypotheses were investigated experimentally by means of some experiments performed in microgravity: the first one, named RUBI (Reference mUltiscale Boiling Investigation), concerned the boiling part and was conducted on board International Space Station between 2019 and 2021. The second one, named ARLES (Advanced Research of Liquid Evaporation in Space), regarded evaporation and was conducted on board a suborbital sounding rocket in 2019. They are unique and high-impact experiments, as they represent the first European boiling experiment in a long-term microgravity environment and the first drop evaporation experiment on a suborbital facility - that allows about 6 minutes of microgravity. Advantages of experiments in space are several: firstly, experimental data in actual weightlessness are collected, to compare with theoretical models and numerical calculations. Furthermore, it is possible to observe some phenomena that are normally masked by gravity. Sometimes - especially in boiling - processes occur slower and at larger length than on Earth, allowing an higher spatial and temporal resolution. All of that contributes to increase the understanding of the physics involved, with implications on both space and terrestrial applications. The above-mentioned experiments were followed closely during their preparation and execution; a huge database of experimental data has been created - it has been used for the present work and will be used in the future. In the case of bubbles, it has been observed that they detach regularly in weightlessness because of the electrode, contrary to what happens in the absence of electric forces. Detachment mechanism, related to high gradient regions of the electric field, has been established. Concerning evaporating drops, an enhancement of the evaporation rate in the presence of the electric field has been observed. Such observation is remarkable, as on ground it is not the case - possibly because gravity and natural convection prevail. An extensive theoretical and numerical analysis of the electric forces acting in the two systems has been performed. These forces were classified with a new criterion, that better clarifies their action on bubbles and drops. Numerical results show good agreement with experimental measurements