On progeny droplets emitted during Coulombic fission of charged microdrops.

The charge level at which a drop undergoes a Coulombic fission is given by the Rayleigh limit. The charge and mass losses from the drop during the fission and the characteristics of resulting progeny droplets, however, remain unpredictable. The charge of a dielectric drop arises from ions, and we have examined the effects of ions on fission-related charge and mass emissions from single levitated microdrops. The ion concentration in a drop was varied through addition of appropriate amounts of an ionophore or ionic liquid. The results show that the mass loss from a drop decreases, while the charge-to-mass ratio of progeny droplets increases as the ion concentration in the drop increases. From these observations we establish that nonuniform ion distributions that exist in a charged dielectric drop play a dominant role in the fission process by influencing the electrical conductivity at the drop surface. We show that the charge-to-mass ratio of progeny droplets is proportional to the conductivity at the surface of the mother drop, which has been calculated from the mobilities of ions and ion concentrations at the surface of the mother drop. By minimizing the Gibbs free energy change associated with a Coulombic fission we deduce that a progeny droplet carries 50% of the Rayleigh limit charge, and from the experimental data we establish that the progeny droplet size varies inversely to the surface conductivity raised to the power of 2/3.