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

Molecular dynamics simulations are used to explore explosive boiling of thin water films on a gold substrate. In particular, water films of 0.7, 1.6, and 2.5 nanometer thickness were examined. Three different surface wettabilities with contact angles of 11 deg, 47 deg, and 110 deg were simulated along with substrate temperatures of 400 K, 600 K, 800 K, and 1000 K. The 11 and 47 deg contact angles were obtained using a Morse interaction potential between the water film and gold substrate while the 47 and 110 deg contact angles were obtained via a Lennard-Jones potential. Evaporation was the first mode of phase change observed in all cases and explosive boiling did not occur until the substrate reached a temperature of 800 K. When explosive boiling was present for all three contact angles, it was consistently shown to occur first for the surface with a 47 deg contact angle and Lennard-Jones potential. These results suggest that explosive boiling onset is strongly dependent on the particularities of the interaction potential. For instance, the Morse potential is smoother when compared to the Lennard-Jones potential, but has more interaction sites per molecule—two hydrogen atoms and one oxygen atom versus one oxygen atom. Thus, even when the water film reaches a higher temperature with the Morse potential, explosive boiling onset is delayed as more interaction sites have to be disrupted. These results suggest that contact angle alone is insufficient and both the interaction strength and the number of atoms interacting at the interface must be considered when investigating trends of explosive boiling with surface wettability.