Molecular-Level Modeling of Interfacial Phenomena in Boiling Processes

The thermophysics of liquidvapor interfaces has long been recognized as a critical element in the physical mechanisms of boiling processes. This article will describe results of recent molecular dynamics simulation studies that explore the structure and stability of liquidvapor interfacial regions using a hybrid analysis scheme that combines new formulations of capillarity theory with molecular dynamics simulations that use similar interaction potentials. Two forms of this type of hybrid scheme have been developed: one for non-polar fluids based on a LennardJones interaction potential and a second specifically for water using a modified treatment of the extended simple point charge interaction potential that accounts for water dipole interactions. The hybrid model has the advantage that the capillarity theory provides theoretical relationships among parameters that govern interfacial region structure and thermophysical behavior, while the companion molecular dynamics simulations allow more detailed molecular level exploration of the interfacial region thermophysics. Predictions of interfacial region structure indicated by this kind of hybrid modeling will be described for pure non-polar and water liquidvapor interfaces and for water with dissolved ionic solutes (i.e., salts). Extension of the methodology to thin liquid films will also be described. Rupture of a free liquid film dictates merging of adjacent bubbles, which is particularly important in nucleate boiling heat transfer, bubbly two-phase flow in small tubes, and the mechanisms that dictate the Leidenfrost transition. To understand the mechanisms of bubble merging in nanostructured boiling surfaces and in nanotubes, it is useful to explore film stability and the onset of rupture at the molecular level. Results obtained with the hybrid model indicate that wave instability predominates as an onset of rupture mechanism for free liquid films of macroscopic extent, but for free liquid films with nanoscale lateral extent (e.g., in nanostructured boiling surfaces), lack of film core stability is more likely to be the mechanism. Predictions of the hybrid models will be compared to results of experimental studies of the effects of ionic solutes on interfacial tension and bubble merging. The implications of the molecular dynamics model predictions for boiling processes in micro-channels and boiling in nanostructured surfaces are also discussed.