Charge Reduction in Electrosprays:  Slender Nanojets as Intermediates

Molecular dynamics simulations were used to study charge reduction in electrosprayed liquids through the formation of slender nanojet intermediates. The dynamics of shape relaxation and disintegration were followed as a function of charge in cylindrical water nanojets containing protonated diglycine molecules. Depending on the overall charge, simulations showed three basic scenarios for nanojet evolution. Moderately charged nanojets reduced to spheres, whereas nanojets charged close to the Rayleigh limit divided into two offspring droplets. Due to the large Coulomb interaction between ions, highly charged nanojets suffered repeated fission until the resulting droplets were charged below the Rayleigh limit. We demonstrated the role of surface fluctuations and Maxwell stress distributions in the disintegration process. The relaxation dynamics of the moderately charged systems to spherical geometry followed a damped oscillator behavior. Compared to neutral water jets, the presence of charges in subcritical nanojets resulted in a stiffer system with longer relaxation times to spherical geometry. Interparticle forces acting between the separating offspring droplets in nanojet breakup were also determined. Due to the increased role of fluctuations in nanojets, the Rayleigh limit was shown to overestimate the maximum charge on stable systems indicating higher nanodroplet production efficiency than one would expect from macroscopic theories alone.