Negative droplets from positive electrospray

Image charge detection has been used to measure the charge and velocity of individual electrosprayed water droplets. With a positive bias on the electrospray needle the majority of the droplets are, as expected, positively charged. However, a small fraction, surprisingly, carry a negative charge. Plausible explanations for the presence of the negatively charged droplets are discussed. In particular, we consider the possibility of the negatively charged droplets resulting from a bipolar fission process where the incorporation of a small negatively charged droplet between two larger positively charged progeny lowers the energy barrier for symmetric fission.