Molecular Dynamics Simulations of Non-Photochemical Laser-Induced Nucleation: Electrolyte Clustering by Nanoparticle Heating

Non-photochemical laser-induced nucleation (NPLIN) is a process where a crystalline phase is formed out of solution by exposure to a laser beam. In NPLIN, the nucleation probability is strongly dependent on the beam intensity and weakly dependent on the wavelength. NPLIN offers a feasible alternative to energy-intensive industrial crystallisation methods. Although several mechanisms have been proposed, little is known about NPLIN at the molecular level. Some theories suggest that nucleation rates are enhanced through the heating of nanoparticles by absorption of electromagnetic energy. In this work, molecular dynamics simulations are performed on the clustering of ions in the vicinity of a heated nanoparticle in an aqueous supersaturated KCl solution. The spherical symmetry of a spherical nanoparticle in solution is exploited by modelling a laterally periodic water column of an initial length of 500 Angstrom enclosed between a planar iron(III) oxide nanoparticle surface and a graphene piston. A cavitation bubble is formed after nanoparticle heating, leading to an increase of clustered ions according to a bond order criterion. The clustering should correspond to nucleation in experimental systems because the clusters satisfy Delta G < 0 for locally valid NPT ensembles. The results corroborate concentration based NPLIN mechanisms as the clustering is visibly induced by local solute evaporation. Pressure based mechanisms appear ineffective because no effects of pressure waves are observed. Thermostatting the graphene sheet does not yield observable dissipation of the thermal energy generated through the nanoparticle heating and at the ion clustering sites and, obstructing completion of the cavitation cycle at a feasible simulation duration. It is suggested to repeat the simulations using a conically shaped system and a piston of higher transverse thermal conductivity.Applied Physic