The In-Flight Breakup of Drops Subjected to Electric and Ionic Fields of DC Corona Discharge

In-Flight ElectroSpraying (IFES) is a novel approach to generate electrically charged micrometer-sized and/or nanometer-sized progeny drops in a controlled manner. This is accomplished in flight, beginning with millimeter-sized parent drops. The break-up of charged drops that is exploited in IFES also occurs (sometimes unwantedly) in conventional electrospraying, electrospinning, and electrospray mass spectrometry. An understanding of what controls the break-up of a parent drop into its progenies is essential in estimating the resultant sizes and charges. The results of experiments on the break-up of electrically charged parent drops into progeny drops are described. The break-up occurs in applied electric (1 kV/cm to 10 kV/cm) and ionic (10¹³/m³ to 10¹⁵/m³) fields that are generated using a DC-corona discharge in a needle-plate configuration. The size, velocity, and charge of mm-sized to nm-sized drops are measured for different initial parent drop sizes. To predict the distribution of the ion space charge, the current density is measured at different positions on the grounded electrode. The effects of the applied electric field strength, the ion concentration and the initial drop surface temperature on the drop break-up are quantified. Several models also are summarized, including simulations of the electrohydrodynamics of the corona discharge, drop charging, drop trajectory analysis, and an electro-capillary stability analysis of the drop's charged surface. The experiments clearly demonstrate that the charge and size distribution of the progeny drops can be controlled by subjecting specific-sized parent drops to different electric and ion fields. The resultant charge of first-generation progeny drops is found to vary as d^3/2, where d ranges from ~ 1 Ì_å_m to ~ 100 Ì_å_m, and is independent of the intensity of the applied electric field and the parent drop size. Similarities of the observed drop in-flight spraying modes with those of conventional electrospraying from capillaries are noted.