Computational and experimental investigation of nanodroplet charging

Summary form only given. An innovative approach to control the droplet size distribution produced by an electrospray was investigated experimentally and numerically. The objective of this effort is to be able to deliver precisely controlled quantities of precursor materials for nanofabrication. The technique uses electron bombardment to charge droplets in flight to a level exceeding the Rayleigh limit, resulting in droplet fission and a modified droplet size distribution with a smaller mean droplet diameter. We developed a theoretical model for the electrodynamics of electrospray droplets injected in an electron beam. The model has been implemented using an unstructured 3d particle-in-cell Monte Carlo methodology (UPIC-MC). Simulations have validated the code by comparisons with analytical charging of isolated droplets. In addition, the electron emission process has been simulated in 3d and results have been compared with experiments. Finally, a parametric investigation of non-moving droplets under various emission conditions provided prediction of the charging process. To test the viability of this technique, an electrospray was used which produces with negatively charged droplets of a room temperature ionic liquid solution. This jet, upon breakup forms a plume which was directed through a stream of electrons produced by a thermionic cathode. Droplet specific charge and mass were determined with a charge detection mass spectrometer sensor allowing a determination of droplet velocity, charge, specific charge, and diameter. Results showed the specific charge distribution broadened with a shift towards larger values. Since the mean droplet charge was found to decrease, the increase in specific charge is evidence of droplet mass decreasing, suggesting that droplet fission is in fact occurring. In addition, the droplet size distribution was found to narrow with a corresponding reduction in mean diameter. Mechanisms other than fission which could explain the reduction in mean droplet diameter, including evaporation of the droplets while in transit over the cathode, are also discussed. In addition to nanofabrication, this work has relevance to plasma charging of aerosols and dust as well.