In situ measurements of the apex electric field in an electrospray emitter using scanning electron microscopy

In-situ observation of an operating ionic liquid electrospray emitter was achieved using a LaB6 transmission electron microscope. The emitter was an electrochemically etched tungsten needle that was externally wetted with the ionic liquid OMIM-BF4. During both positive and negative emission solid dendritic structures were created and left behind at the emission sites. These structures were seen to grow on timescales of a few minutes. It is possible that depletion of one ion species (cation or anion) from the emission region drastically changed the physical behavior of the propellant left behind, possibly resulting in solidification of a new chemical substance at the emission site. This solidification may be independent of any interaction with the metallic substrate. Such observations have not been witnessed in electrospray research, and though the underlying cause is presently unknown the molecular structure of the ionic liquid most likely plays a role. A final observation was that a majority of the emission sites during BF4 and OMIM + operation were at locations along the shaft of the tungsten needle instead of the cone apex.