Measurements of the charge and size of heptane droplets generated by electrostatic sprays showed that the droplet charge-to-volume ratio is a monotonically decreasing function of size. In the useful range of electrospray operation, characterized by droplets smaller than the size of the orifice from which the liquid is issued, it was found that the larger were the droplets the closer they were to the Rayleigh limit. In particular, when droplets had charging levels between 70% and 80% of such limit, they were observed to rupture because the repulsive force due to surface charge evidently overcame surface tension. The rupture phenomenon, here termed Coulomb fission, was also captured in microphotographs that typically showed a droplet with one or two, diametrically opposed, conical protrusions terminating in a fine jet ejecting a stream of much smaller, apparently equisized offsprings. The process appeared swift and, yet, well ordered, quite different from the common view of a violent, convulsive explosion. Corroborating evidence on the disruption pattern was also gathered by quantitative measurements of the evolution of the droplet size distribution in evaporating sprays using phase Doppler anemometry (PDA). Implications of these findings are finally discussed in the context of a particular application of electrostatic sprays, electrospray ionization, a technique that is revolutionizing the mass-spectrometric analysis of large biomolecules.

An experimental study has been performed on the structure of an electrostatic spray of monodisperse droplets. Such a spray is established when a liquid with sufficient electric conductivity and moderate surface tension, in the present case heptane doped with an antistatic additive, is fed through a small metal tube maintained at several kilovolts relative to a ground electrode a few centimeters away. The liquid meniscus at the outlet of the capillary takes a conical shape under the action of the electric field, with a thin jet emerging from the cone tip. This jet breaks up into charged droplets that disperse into a fine spray. Flash shadowgraph of the breakup region showed that the jet initially breaks into droplets of bimodal size distribution by varicose wave instabilities. The spray monodispersity is established farther downstream by a segregation process of electrostatic and inertial nature that confines the bulk of the mass flow rate (97%) and 85% of the total current in a core of nearly monodisperse primary droplets, with the remainder in a shroud of satellites. Droplet size, axial velocity, and concentration were measured throughout the spray by phase Doppler anemometry (PDA). The complementary use of these measurements permitted the determination of the electric field via the spray momentum equation. It was found that droplets are ejected from the jet at a relatively high velocity in a region characterized by a very intense electric field. They maintain this velocity farther downstream because of inertia, even though the field is precipitously decreasing, and ultimately decelerate under the action of the drag force and a progressively weaker electrostatic force. Velocity and concentration fields were shown to be self-similar. Comparison between the external field, due to the potential difference applied between the electrodes, and the space charge field shows that the droplet axial motion is driven primarily by the external field, whereas the droplet radial motion and, consequently, the jet lateral spreading, is controlled primarily by the space charge field. The latter is typically at least one order of magnitude smaller than the external one, except at off-axis locations near the breakup region of the spray, where the two fields can be comparable. The droplet charge distribution was also determined via the spray momentum equation and the simultaneous measurements of droplet size and velocity in a region where droplets experience negligible acceleration. The charge distribution was found to be narrow, with a ratio of standard deviation over mean of 0.15.

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The development, fabrication, and testing of a compact multiplexed system of electrosprays are presented with the dual goal of increasing by orders of magnitude the liquid flow rate to be dispersed and of retaining the quasimonodispersity of the generated droplets. The system was microfabricated as an array of nozzles etched in silicon, with a density of 250 sources/cm 2 . Although the operation of a single electrospray is rather forgiving with respect to the electrode geometry, successful performance of the multiplexed system is critically dependent on a careful selection of the electrode configuration, which in the present work entails an extractor electrode mounted at a distance from the spray sources that is comparable to the distance between sources (on the order of 0.5 mm). The electrode has the dual function of limiting electric field cross-talk between neighboring sources and minimizing space charge feedback from the spray cloud. Measurements of current and droplet size as a function of flow rate and of droplet size distribution using ethanol demonstrated that the system may be optimized to produce uniform droplets simultaneously from all parallelized electrosprays, each one operating as an isolated spray in the quasimonodisperse cone-jet mode. Ease of operation and uniformity in size from spray to spray require strategies to increase the pressure drop in the liquid flow path and/or to uniformize the electric field at the spray sources.

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