Fundamental Roles of Nonevaporating Film and Ultrahigh Heat Flux Associated With Nanoscale Meniscus Evaporation in Nucleate Boiling

Abstract

The three-phase moving contact line present at the base of a bubble in nucleate boiling has been a widely researched topic over the past few decades. It has been traditionally divided into three regions: nonevaporating film (order of nanometers), evaporating film (order of microns), and bulk meniscus (order of millimeters). This multiscale nature of the contact line has made it a challenging and complex problem, and led to an incomplete understanding of its dynamic behavior. The evaporating film and bulk meniscus regions have been investigated rigorously through analytical, numerical and experimental means; however, studies focused on the nonevaporating film region have been very sparse. The nanometer length scale and the fluidic nature of the nonevaporating film has limited the applicability of experimental techniques, while its numerical analysis has been questionable due to the presumed continuum behavior and lack of known input parameters, such as the Hamaker constant. Thus in order to gain fundamental insights and understanding, we have used molecular dynamics simulations to study the formation and characteristics of the nonevaporating film for the first time in published literature, and outlined a technique to obtain Hamaker constants from such simulations. Further, in this review, we have shown that the nonevaporating film can exist in a metastable state of reduced/negative liquid pressures. We have also performed molecular simulations of nanoscale meniscus evaporation, and shown that the associated ultrahigh heat flux is comparable to the maximum-achievable kinetic limit of evaporation. Thus, the nonevaporating film and its adjacent nanoscale regions have a significant impact on the overall macroscale dynamics and heat flux behavior of nucleate boiling, and hence should be included in greater details in nucleate boiling simulations and analysis.