(46a) Characteristics of Droplet Explosions Studied with Non-Equilibrium Molecular Dynamics Simulations

In flame spray pyrolysis, functional nanoparticles are produced by dissolving a metal-containing precursor in a combustible solvent and spraying the mixture into a flame. In that process, the droplets of the precursor solution undergo a cascade of microexplosion events that originate from within the droplet or close to its surface [1-3]. Interestingly, the quality of the obtained nanoparticles is strongly related to the occurrence of such microexplosions [2,3]. However, to date, the causes for these microexplosions are not well understood, mainly because experimental investigations of the interior of the droplets are infeasible. It is suspected that the microexplosions are linked to internal superheating in the droplet [1-3].

In this work, we have developed a non-equilibrium molecular dynamics simulation scenario to investigate characteristics of droplet explosions on a fundamental level. The simulation scenario was set up and tested considering the Lennard-Jones truncated and shifted (LJTS) model fluid, since it is a realistic model of the qualitative behavior of fluids and computationally inexpensive. In the simulation scenario, a droplet of a pure fluid is first equilibrated with its coexisting vapor. Then, in the middle of the droplet, a thermostat is activated that imposes a high temperature. Several characteristics of the temporal evolution of the system are then tracked, e.g. the position of the droplet-gas-interfaces, the number of particles in the liquid phase, and the radius of gyration of the droplet. Using that simulation scenario, a series of simulations was carried out in which the set temperature of the thermostat was varied. For low set temperatures, the droplet continuously evaporates, following the d² law of droplet evaporation [4]. By contrast, for high set temperatures, a vapor bubble nucleates inside the liquid droplet, the droplet expands and eventually breaks up. This process also involves several oscillations of the droplet size. These qualitative results help to identify causes for droplet microexplosions.

[1] N. Jüngst et al., Exp. Fluids 63 (2022) 60.
[2] C. Rosebrock et al., AIChE J. 62 (2016) 381-391.
[3] H. Li et al., Combust. Flame 215 (2020) 389-400.
[4] F. Meierhofer and U. Fritsching, Energ. Fuel. 35 (2021) 5495-5537.