Explosions of nanodroplets studied with molecular dynamics simulations

Explosions of droplets that are caused by superheating of the liquid phase occur in many combustion processes but are difficult to investigate experimentally. We have studied this process for nanodroplets using non-equilibrium molecular dynamics simulations. Starting from an equilibrium state in which a spherical droplet is surrounded by a vapor phase, a local thermostat is used to impose a high temperature in a small control volume in the droplet center and the following process is studied for varying set temperatures. The fluid is modeled using the Lennard-Jones truncated and shifted potential. Depending on the set temperature, three different system responses were observed: (i) Low set temperatures lead to a shrinking of the droplet due to evaporation that follows the well-known d(2) law. (ii) At intermediate set temperatures, a vapor bubble emerges in the droplet center and the liquid phase is formed into spherical shell that expands as the bubble inside of it grows. However, that spherical shell is only temporarily stable and eventually breaks apart. (iii) For high set temperatures, the abrupt and violent formation of the vapor bubble leads to an immediate breakup of the droplet. For case (ii), unexpected phenomena were observed. Oscillations in the diameter of the vapor bubble surrounded by the liquid film occurred. In some simulations, small holes formed temporarily in the liquid shell during its expansion, which closed again over the course of the simulation. Moreover, for one specific set temperature, a transition of the spherical droplet shell into a torus-like object was observed.