Electrospraying insulating liquids via charged nanodrop injection from the Taylor cone of an ionic liquid

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Charge is injected into the bulk of an insulating liquid in the form of nanodrops produced by an immersed Taylor cone of an ionic liquid. The charge then drifts onto the insulator surface, destabilizing it and leading to the formation of an electrified jet that atomizes into approximately monodisperse micron size insulator drops. The approach is similar to those previously based on field injection of ions from sharp tungsten tips, but the continuous renewal and self-sharpening of the liquid charge-injector permits long-term stable operation. Using heptane as the insulator and 1-ethyl-3-methylimidazolium-BF4 as the ionic liquid we produce approximately monodisperse drops with average diameters ranging from less than 4 up to 20 μm, injecting in some cases as little as 0.0002% by volume of ionic liquid. No fundamental limitation restricting the possibility of forming even smaller drops is apparent. The scaling law of Kim and Turnbull [“Generation of charged drops of insulating liquids by electrostatic spraying,” J. Appl. Phys. 47, 1964 (1976)] where the drop diameter varies as the 2/3 power of the liquid flow rate and the −2/3 power of the spray current is confirmed, implying that the drops are on the average charged to 50%–60% of the Rayleigh limit.