Molecular dynamics of nanodroplet impact: the effect of molecular models
Electrospraying of nanodroplets modifies the target surface by sputtering its atoms and amorphizing the outer layers. Collisions of droplets of diameters between 1 nm and 1 m are not yet totally understood. Previous experimental studies have been performed and compared with Molecular Dynamics simulations using the pseudo-atom model (PA) for the droplets. This PA model considers each liquid molecule as a single particle with the same molecular mass. In this study a new atomic/molecule model (AM) which contemplates all atoms and bonds of the droplet is presented. Simulations of the collision of a 5 nm diameter droplet for formamide and EMIMIm are performed using both models. Droplet initial kinetic energies between 3 KeV and 15 KeV are tested. Results show that PA model collisions transfer almost all of their initial kinetic energy to the slab. Temperature fields after the collision are much higher for this model. Furthermore, highly energetic EMIMIm droplets pseudo-atoms penetrate further into the target whereas atoms in the AM model do not. The ratio of Silicon atoms sputtered and the amorphized region depth for the PA models are larger than for the atomic/molecule, which compare well to experimental results. The AM model is more accurate, especially when simulating droplets of high atomic mass for which the PA model particles are very energetic. Energy consumed to break bonds in the AM model is negligible in comparison with the droplet initial kinetic energy. Simulations show that unbreakable bonds can be used in the AM models obtaining almost identical results.