Atomistic Modeling of Jet Formation in Charged Droplets

The first atomistic simulations that reveal the mechanism of Rayleigh fission are presented. It is demonstrated that simple ion or macroion ejection takes place through droplet deformation from a spherical into a distinct "tear" shape that contains a conical protrusion. We assert that the latter state is a free-energy minimum along an order parameter that measures the degree of droplet asphericity. The charged droplet's long-time evolution proceeds by alternating between the two minima above and below the critical value that are reached through solvent evaporation and ion ejection, respectively. For the first time, this mechanism allows one to explain the nature of the progeny droplets and the percentage of charge lost during fission. The cone half angle is estimated and found to be in good agreement with the value predicted from the solution of the electrostatic equation for the dielectric liquid. It is found that the conical deformation is independent of the effect of electrohydrodynamic forces reported in experiments. Contrary to the experimental observations of two diametrically opposite jets for droplets suspended in the electric field, we find that a single jet is formed at the Rayleigh limit. The study provides insight into the mechanism of capture of a macroion in jets appearing in electrospray ionization mass spectrometry (ESI-MS) experiments and may explain the tolerance of the ESI-MS spectrum to salt contamination of the sample.