Electrosprays of highly conducting liquids: A study of droplet and ion emission based on retarding potential and time-of-flight spectrometry

Electrosprays of highly conducting liquids operated in the cone-jet mode produce charged nanodroplets of controllable size and molecular ions. The study of this electrospraying regime is challenging due to the lack of experimental techniques for probing these nanometric systems, and the higher complexity of the physics associated with the onset of ion field emission and self-heating. Jet parameters in the breakup region such as its radius, velocity, potential, and electrification level are key for understanding the formation of droplets and emission of ions, and useful to validate numerical models of cone jets. In the case of micron-sized jets, these quantities can be determined with the values of the retarding potentials and mass-to-charge ratios of the droplets produced by the breakup. This article uses this technique to investigate the parameters of nanometric jets. Retarding potential and mass-to-charge distributions of the beam are measured with retarding potential and time-of-flight analyzers operated in tandem. This combination makes it possible to differentiate between droplets of similar mass-to-charge ratios which, unlike in the case of micrometric jets, are needed to apply the technique. Aside from the jet parameters, the experimental characterization also reveals with great detail the composition of the beam, which includes primary ions emitted from the jet breakup; ions resulting from the desolvation of primary ions; stable primary droplets produced at the breakup; smaller droplets resulting from the Coulomb explosion of unstable primary droplets; and small primary droplets that evaporate a significant fraction of their charge in flight. An analysis of the breakup, parametrized by dimensionless numbers, explains this complexity. Although the experimental characterization only studies the electrosprays of the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide, the analysis is general and can be used to understand the beams of other highly conducting liquids.

Figure 1

Experimental setup: electrospray source, vacuum chamber, and detectors.

Figure 2

Retarding potential (electrostatic mirror) and time-of-flight analyzers operated in tandem.

Figure 3

Time-of-flight curves of the whole beam, for several beam currents and two emitter temperatures.

Figure 4

Retarding potential curve illustrating multiple ionic peaks. Red and black traces are same spectra with different vertical scales.

Figure 5

Retarding potential distributions for several beam currents at $21{}^{\circ }\mathrm{C}$. Like in all experiments reported in this article the emitter potential is ${\varphi }_E=1690$ V.

Figure 6

Time-of-flight spectra at different retarding potentials (see black dots in Fig. 5) for $I_B=230$ nA, $21{}^{\circ }\mathrm{C}$.

Figure 7

Average mass-to-charge ratio vs retarding potential of droplet populations and ion peak ${\text{P}}_1$ from spectra in Fig. 12. Primary droplets and ${\text{P}}_1$ ions are emitted from the jet breakup.

Figure 8

Average mass-to-charge ratio vs retarding potential of primary droplets and ion peak ${\text{P}}_1$, for several beam currents at $21{}^{\circ }\mathrm{C}$.

Figure 9

Critical droplet radius $R_D^{*}$ normalized with the radius of the jet, as a function of the Taylor $\mathrm{\Psi }$ and Ohnesorge numbers $\text{Oh}=J^{-1/2}$; and Rayleigh limit of the droplet with the mass-to-charge ratio of the jet.

Figure 10

Droplet radius normalized with the critical droplet radius $R_D^{*}(J,\mathrm{\Psi })$, for a beam current of 230 nA at $21{}^{\circ }\mathrm{C}$ and three different mechanisms: equipotential breakup $R_{\varphi }$; Rayleigh limit $R_{\mathrm{Ray}}$; and ion evaporation limit $R_{\text{IFE}}$, with $E_{\text{IFE}}=1.2\text{eV}$.

Figure 11

Retarding potential curves for several beam currents at $50{}^{\circ }\mathrm{C}$.

Figure 12

TOF spectra for a 255-nA beam at $50{}^{\circ }\mathrm{C}$, and average mass-to-charge ratios and standard deviations of the droplet populations.