Acoustothermal Atomization of a Refrigerant Liquid Nanofilm: Significantly High Heat Flux and Superheating.

We carry out all-atom molecular dynamics (MD) simulations for capturing the acoustothermal atomization of a refrigerant (Novec 7000) liquid nanofilm placed on a vibrating substrate. Like similar studies on water and refrigerant nanofilms, our findings also reveal three separate regimes of atomization with progressively increasing acoustic energy input: evaporation-driven atomization, nucleate-boiling-driven atomization, and film-boiling-driven atomization. Unlike previous studies, however, we cannot distinguish between the evaporation-driven and nucleate-boiling-driven atomization regimes from the time evolution of the number of atomized molecules; rather, this differentiation can only be made by studying the corresponding time evolution of the film temperature and the temporal variation of bubble volume. Similarly, unlike the case of water and other refrigerant liquids, the maximum of the film temperature during film-boiling-driven atomization can become comparable to or even exceed the maximum film temperature during nucleate-boiling-driven atomization. This stems from a rapid early-time acoustic energy deposition (for conditions that lead to film boiling), while the molecular nature of Novec 7000 and the associated molecule-molecule interactions enforce a much slower coalescence of vapor cavities into a continuous insulating vapor layer. Also, these maximum temperatures signify an unprecedentedly high degree of superheat for a refrigerant liquid. Furthermore, the rise in the film temperature occurs much faster for the case of film-boiling-driven atomization, ensuring that the corresponding heat flux (nearly as high as 6.9 GW/m2 and comparable to the heat flux observed in the atomization of water nanofilms) can be overwhelmingly greater (at early stages of atomization) than the heat flux associated with nucleate-boiling-driven atomization. Finally, we show that the residual number of Novec 7000 molecules (left after atomization) first scales inversely with the parameter dictating the dimensionless acoustic energy input, and then becomes constant with this parameter, indicating the presence of a stable, nonatomizable layer of Novec 7000 nanofilm.