Controlling Mineral Disintegration in Charged Water Microdroplets by Added Transition Metal Ions

In this work, we investigate the effect of electrolyte composition on mineral fragmentation in electrosprayed aqueous microdroplets. Quartz (SiO2) and corundum (α-Al2O3) suspensions containing transition-metal ions in different oxidation states (Fe2+/Fe3+, Cu+/Cu2+, and Cr2+/Cr3+), as well as representative monovalent (Na+, K+) and divalent (Mg2+, Ca2+) cations, were electrosprayed under identical conditions. Among the tested transition-metal salt formulations, the lower-oxidation-state systems were associated with fragmentation at lower applied voltages and smaller nanoparticles, whereas the higher-oxidation-state systems required higher applied voltages for comparable fragmentation. Similar trends were observed across all transition-metal cations (Fe2+/Fe3+, Cu+/Cu2+, Cr2+/Cr3+) on both mineral systems. The generality of this approach, where a foreign ion enhances disintegration efficiency, was further supported by the efficient formation of silica nanoparticles in the presence of the imidazolium-based ionic liquid, BMIM-Cl. Finite-element simulations coupling electrostatics with two-phase flow reveal that moderate volume charge densities (∼10 C·m–3) sustain asymmetric “rough” Coulombic fission with prolonged droplet–mineral contact, whereas high charge densities (50–100 C·m–3) trigger rapid “fine” fission into progeny droplets, dissipating electrostatic energy before effective mineral breakage. These results demonstrate that electrolyte composition is an experimental variable for tuning microdroplet-mediated mineral disintegration and motivate further investigation of the physicochemical factors underlying the observed trends.