Ion and Nanodrop Injection from Taylor Cones into Gases and Liquids
Fernandez de la Mora, Juan
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When sufficiently charged, the interface between a conducting liquid and an insulator (vacuum, gas, liquid) becomes unstable and forms sharp conical tips (Taylor cones) which inject liquid into the insulator. This injection most often takes the form of a micro-jet issuing from the tip of the Taylor cone. The physics of this cone-jet is approximately understood. In particular, the larger the electrical conductivity K of the liquid and the smaller its flow rate Q pushed through the cone-jet, the smaller the jet radius R. However, the process of jet shrinking with increasing K does not go forever. When K reaches values in the range of 1 S/m, R may become as small as 5 nm. This leads to electric fields strong enough for ions dissolved in the liquid conductor to be field-evaporated through the interface, resulting in a mixed regime with simultaneous ejection of ions and drops. An extreme behavior when only ions and no drops are formed has been known for decades in the case of positively charged liquid metals exposed to vacuum. The subject of our enquiry is whether a transition takes place between the convex drop-emitting cone-jet and the presumably concave tip emitting ions alone. We have studied this presumed transition under a variety of circumstances It is better probed with electrolytes than with liquid metals, as the latter have conductivities many orders of magnitude higher than the transitional range K~1 S/m. It is not readily studied when either a liquid metal or an electrolyte is surrounded by a gas because the evaporated ions produce electrical breakdown turning the insulating gas into a conductor. One line of research therefore involves the study (by time of flight mass spectrometry) of highly conducting electrolytes (including molten salts or ionic liquids) in a vacuum. In another approach we substitute the gas by a dielectric liquid, and explore whether or not ions or nanodrops are injected into an insulating liquid. Although the dielectric liquid alters drastically the situation through space charge effects limiting the current, we observe the production of nanodrops in the 5-10 nm size range, as well as ion injection. The purely ionic regime has been encountered with ionic liquids in vacuum, but not yet in insulating liquids.