The investigation of explosive boiling of thin water films is of importance to many applications including nanoparticle-mediated cancer hyperthermia using plasmonic heating, laser ablation and the examination of protein and DNA folding dynamics. Molecular dynamic (MD) simulations are an important tool for understanding the peculiarities of the ultra-fast phase transition of water at small scales. Several MD studies have been published on this topic. A recent report by Wang, et al. (Intl J Heat Mass Trans 2019 132:1277) suggests that classical nucleation theory breaks at small scale. Specifically, they report that below a threshold thickness, explosive boiling of the water films occurs earlier on hydrophilic surfaces as compared with hydrophobic surfaces. This finding motivates the present work. Thus, the objective of this paper is to revisit the explosive boiling of ultra-thin water films to better understand the physics governing phase-change at this scale.

To accomplish this objective, the simulations reported in the aforementioned study have been repeated for similar conditions including a water thickness of 2.5 nm, but a higher wall temperature of 1000K. In addition, a thinner film of 0.7 nm was also investigated. The wall was a gold substrate that consisted of 1995 gold atoms arranged in nine layers with a 111 lattice orientation while the 2.5nm and 0.7nm water films consisted of 1936 and 528 water molecules respectively, in an arrangement consistent with the Ih structure of ice. In all simulations performed the SPC/E water model was utilized and gold-gold interactions were modeled by the embedded atom method. Three primary sets of parameters for the water-gold interactions were used. Two sets of parameters made use of the 12-6 Lennard-Jones (LJ) and Coulombic potentials to make the surface explicitly hydrophobic and hydrophilic as described by Wang. The last set of parameters made use of a Morse potential described by Berg, et al. (J. Chem. Theory Comput. 2017 13:5610), which better agreed with density functional theory and reproduced the energy configuration of water on gold. After an equilibration time of 50ps, the temperature of the substrate was rapidly increased from 298K to 1000K in a time frame of 3ps. The temperature was then maintained at 1000K.

The results show that for the 0.7nm thickness all films experience evaporation as the first mode of heat transfer. Both the hydrophilic and Berg surfaces experience fast evaporation until dry out. Their temperatures abruptly rise to 700K then drop slowly to 600K at the end of 400ps simulation time. In contrast, the 0.7nm water film on the hydrophobic surface first evaporates at a slower rate. However, at a certain point, a large cluster of remaining water molecules are ejected from the surface, which resembles explosive boiling and supports nucleation theory. The temperature of this film rises asymptotically to 500 K. This difference in temperature rise indicates the thermal resistance across the water/gold interface is much smaller for the hydrophilic film than hydrophobic film.

All films of 2.5nm thickness experience explosive boiling. The hydrophilic and Berg surfaces asymptotically approach a temperature of 600K, with the former boiling first. However, the water film in the hydrophobic case is ejected from the substrate before the temperature reaches 1000K. To mitigate this, the parameters for modeling the hydrophobic surface were altered as compared with those from Wang in order to make the surface less hydrophobic. With the new set of parameters, early ejection is avoided and the temperature asymptotically approaches 530K. The onset of explosive boiling in these hydrophobic cases is later than both the hydrophilic and Berg cases, which go against classical nucleation theory. However, the delay in explosive boiling is not unexpected since the lower temperature rise indicates lower heat transfer rates to the water film. These results suggest the need for further investigations of the phase transition of water at this scale to better understand the main physical mechanisms at play. They also highlight the sensitivity of simulation results to interface force-field parameters.


Molecular Dynamics Simulation of Ultra-Fast Phase Transition in Water Nanofilms

Division / Sponsors: 
Heat Transfer Division (HTD), Fluids Engineering Division (FED)

Heat Transfer Summer Conference
Paper Proceedings: https://asmedigitalcollection.asme.org/HT/HT2020/volume/83709

18th International Conference on Nanochannels, Microchannels, and Minichannels
Paper Proceedings: https://asmedigitalcollection.asme.org/ICNMM/ICNMM2020/volume/83693

Fluids Engineering Division Summer Meeting
Paper Proceedings: https://asmedigitalcollection.asme.org/FEDSM/FEDSM2020/volume/83716

HT-ICNMM2020 - Virtual Conference, SHTC - Virtual Conference, FEDSM2020 - Virtual Conference

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