Titolo della tesi: EVAPORATION AND CONDENSATION OF MICRO AND NANO DROPLETS
Phase transitions are among the most widespread processes both in nature and in technological applications. Particular interest is devoted to droplet evaporation and condensation in order to study and understand their mechanics and thermodynamics. These processes exert a significant influence on a variety of technologies, spanning from chemical and energy industries to medical diagnostics. A thorough understanding of the droplets' phase change is crucial, as they still present open questions due to their high level of complexity, especially when considering droplets lying on a solid surface (sessile droplets).
The phenomena comprise several coexisting aspects, including: mass transport across the vapour-liquid interface, radius or contact line dynamics and, in the presence of transported species, the formation of patterns on the substrate.
In addition to the aforementioned aspects, it is essential to consider the inherent multi-scale nature of the process in the presence of fluid-substrate interaction. This fundamental complexity presents a significant challenge to achieving a comprehensive understanding of the phenomenon, both from an experimental and a numerical perspective.
From one perspective, experimental studies are constrained by the small space-time scales, which render acquiring reliable quantitative measurements challenging. Conversely, numerical simulations are typically constrained to macroscale analyses or systems with dimensions of a few nanometres, thereby precluding the possibility of mesoscale investigation.
The challenge of comprehending the phenomena in their entirety, coupled with the advent of advanced technologies offering enhanced efficiency and compact design, has prompted a surge in research into the phase change of droplets at the mesoscale, with dimensions below a few micrometres.
This thesis employs a continuum diffuse interface model to investigate the evaporation and condensation processes of submicrometric-sized droplets. The integration in the Navier-Stokes equations allows for a comprehensive description of the dynamics and an in-depth understanding of the driving forces behind these processes under various environmental conditions.
This modelling framework provides an accurate representation of the behaviour without the limitations imposed by simplifying assumptions and external flux models, such as those used in Volume of Fluid (VOF) methods, or the high computational costs associated with Molecular Dynamics (MD) simulations. In the context of sessile droplets, conventional macroscale frameworks based on sharp interface approaches require the use of slip models to address the inherent paradox of a moving contact line with a no-slip boundary condition on the wall. In contrast, the diffuse interface approach circumvents this issue even when changes in wetting conditions occur.
The study examines several phenomena, including the thermally induced Marangoni effect, which generates recirculation zones in both the liquid and vapour phases, and the Leidenfrost effect, which influences the dynamics of the contact line between the three phases and significantly varies the mass fluxes.
The principal objective of this study is to gain a deeper understanding of droplet evaporation and condensation dynamics, particularly in the presence of a solid surface, through numerical simulations employing a diffuse interface model based on the van der Waals theory of capillarity. This will enable the capture of the entire multiscale process and the elucidation of the phenomena that govern droplets' internal flows.
Furthermore, in addition to the analysis of pure fluid phase transition, the research is extended to consider the presence of an inert species transported by the fluid and confined within the liquid phase. This is achieved by extending the van der Waals model through the introduction of a solid-fluid-species free energy functional.