Diameter and charge of the first droplet emitted in electrospray

Abstract

The first droplet produced by a low-conductivity pendant/sessile droplet subject to a strong electric field is particularly important at the fundamental level because, in contrast to steady electrospray phenomena, its ejection entails complex charge relaxation and electrokinetic processes. Besides, it is technologically relevant because of its very small diameter and large electric charge per unit volume. In this work, we present an experimental technique to measure with unprecedented accuracy the diameter of the droplet and to determine for the first time its electric charge. We discuss both the advantages of our technique over possible alternatives and the limitations of the method. The proposed method is applied to two alcohols with electrical conductivities of the order of a few μS/m. The high sensitivity of our experimental technique allows us to determine the influence of both the magnitude and the polarity of the applied voltage on the size and charge of the ejected droplet. The electric charge of the first-emitted droplet lies in the interval 0.51≲q/qR≲0.66 (qR is the Rayleigh limit of charge) for the two liquids analyzed. These experimental values are slightly larger than those obtained from theoretical predictions. The value of q/qR for the first droplet is very relevant because it can be regarded as an upper bound of those of the droplets subsequently emitted in the cone-jet mode of electrospray.