Free liquid films bounded on two sides by vapor are of particular interest in nucleate boiling process because the merging of bubbles is ultimately associated with the rupture of a liquid film separating the vapor inside the bubbles. The stability and conditions for the free film rupture are therefore of central importance to the merging process because they dictate whether coalescence is likely to occur. Most previous studies of free film stability have focused on a continuum model of wave phenomena at the interfaces of the film. In the investigation summarized here, molecular dynamics (MD) simulation was used to explore the stability of free films of liquid argon surrounded by saturated argon vapor. The MD simulations predict that the wave perturbation or density fluctuation induced by the thermodynamic intrinsic instability can lead to rupture of the liquid film, as its thickness decreases below a critical value. It further predicts that the critical film thickness derived from density fluctuation increases with system temperature. These predictions were also compared with those of conventional interface wave perturbation analysis. The result of this investigation suggests that during bubble merging, thermodynamic instability of the liquid phase may play a role in the onset of film rupture as the liquid film separating the bubbles gets progressively thinner.

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