A nanoscale view of the origin of boiling and its dynamics

Abstract Boiling has the peculiarity of being involved both in frontier technological applications, such as nano cooling, and in everyday life activities, such as food cooking. For that reason, everyone is very familiar with this kind of phase transition. Associated with the empirical knowledge of the process, despite tens of years of scientific exploration, there is yet no adequate physical understanding of the phenomenon and its profound nanoscale origins. Many controversial experimental observations well fit into such a scenario. The lack of fundamental understanding of the process has a great impact on the technological development of thermal science, especially in terms of a quantitative prediction of the physical parameters. The central issue lies in the multiscale and non-equilibrium nature of boiling itself, where nucleation and bubble dynamics must coexist in a single description far from thermodynamic equilibrium and linked to length and time scales, unreachable by the present measurement tools. To date, theory, simulations, and experiments have described the different scales of boiling always compartmentalised and blurred by many uncontrolled parameters. Therefore, a holistic description encompassing the whole aforementioned range of spatial and temporal scales is still lacking, even though strongly desired. Here we show for the first time that a continuum model based on fluctuating hydrodynamics and the diffuse interface approach is able to describe the boiling phenomenon from the stochastic nucleation up to the macroscopic bubble dynamics. The approach covers, with a modest computational cost, the mesoscale area from nano to micrometers, where most of the controversial observations related to the phenomenon originate from. In particular, the role of wettability in the macroscopic observable of boiling is elucidated. In addition, by comparing the ideal case of boiling on ultrasmooth surfaces with a chemically heterogeneous wall, our results will definitively shed light on the puzzling low onset temperatures measured in experiments. Sporadic nanometric spots of hydrophobic wettability will be shown to be enough to trigger the nucleation at low superheat values, significantly reducing the temperature of boiling onset, in line with experimental results. The proposed mesoscale approach constitutes the missing link between macroscopic approaches and molecular dynamics simulations, and will open a breakthrough pathway toward accurate understanding and prediction.