Hybrid modeling of interfacial region thermophysics and intrinsic stability of thin free liquid films

Abstract

The film rupture process that dictates merging of adjacent bubbles 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 onset of rupture at the molecular level. This paper reports the results of such an investigation using a hybrid analysis scheme that combines a new formulation of capillarity theory for free liquid films with molecular dynamics (MD) simulations that use similar interaction potentials. Two forms of our molecular film capillarity theory are developed here: one for non-polar fluids based on a Lennard-Jones interaction potential, and a second specifically for water using a modified treatment of the SPC/E 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 film structure and thermophysical behavior, while the companion MD simulations allow more detailed molecular level exploration of the film thermophysics. Results obtained with the hybrid model indicate that wave instability predominates as an onset of rupture mechanism for liquid films of macroscopic extent, but for free liquid films with nanoscale lateral extent (in, for example, nanostructured boiling surfaces), lack of core stability is more likely to be the mechanism. The implications of these predictions for film rupture and bubble merging in nanostructured surfaces and nanotubes are examined in detail.