Effect of Aspect Ratio on the Wettability and Electrospray Properties of Porous Tungsten Emitters with the Ionic Liquid [Emim][Im

Abstract

Porous tungsten emitters of different aspect ratios, produced by a novel electrochemical etching process, are studied by angle-resolved mass spectrometry and retarding potential energy analysis, which provides information on the mechanisms of ion field evaporation and spatial distributions of the emitted ions and droplets. In the present study, the ionic liquid [Emim][Im] (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) is sprayed from two externally wetted porous tungsten emitters. Near field current and mass flow measurements show that while both emitters operate in a mixed ion-droplet mode, the longer 3 mm emitter has a slower liquid flow rate than the shorter, 2 mm emitter. The mass spectrometric analysis shows that the 3 mm emitter favors the production of smaller ions, while the mass distribution shifts to larger clusters with the use of the 2 mm emitter. The ion energy analysis suggests that all the observed ions and droplets, from both emitters, have energies near that of the emitter potential, seemingly indicating that they are all formed in the neck region of the Taylor cone. The combined near field and mass spectrometry results are consistent with the observation that the shorter emitter supports a larger liquid flow rate to the tip than that produced from the longer emitter, leading to emission that contains a larger fraction of droplets versus ions, and also produces droplets of a larger size.