Electrospray pulsation: A diagnostic to understand cone-jet stability and minimum flow

Detailed current and flow rate measurements obtained during pulsation mode nano-electrospray of ethylene glycol solutions are presented. High temporal resolution current measurements reveal for the first time that current is observed at the electrode immersed in the electrospray fluid, even when there is no observed spray. This current, believed to be the charging current associated with electrode and meniscus polarization, is dependent upon the voltage applied to the electrospray system. Quantitatively, the total charge transfer that is observed during the non-spray period of the pulsation approximates to that required to obtain electrostatic equilibrium in the conical meniscus, and is relatively independent of the fluid conductivity. A linear relationship exists between this charging current and the observed pulsation frequency. Pulse shape, indicated by the parameters of spray current rise time, fall time, and spray current pulse duration, are observed to be independent of frequency for a given solution. The total charge lost during a single pulsation event is significantly larger than the charge transfer observed during the meniscus replenishment phase of the pulsation. For the solvent tested, the rate at which meniscus charging takes place is linearly dependent upon solution conductivity, and thus appears to scale directly with the electrical relaxation time.