UNDERSTANDING ELECTROSPRAY REGIMES

It is widely accepted that by applying high voltage to a liquid pumped through a capillary, the liquid takes the shape of a cone (Taylor cone1) and a liquid jet is ejected through its apex. The charged droplets formed by the break up of the jet suffer successive solvent evaporation followed by Rayleigh fission events2,3 and, eventually, gas-phase ions. It is frequently assumed that the Taylor cone forms as soon as the voltage reaches a value that can sustain electrospraying; this cannot be farther from the truth. This page shows several movies captured with a fast camera (exposure time ~7 ns), which expose events unavailable to the naked eye or a regular camera.

The easiest way to reveal the fast liquid dynamics is by measuring the spray current with an oscilloscope. The plot on the right shows current measurements (similar to those described by Juraschek and Röllgen4, corresponding to three applied voltages (green: 2750 V, black: 2950 V, and red: 4050 V). Only the red waveform corresponds to the cone-jet regime; the green and the black curves correspond to the burst and the pulsating regimes, respectively. Each current oscillations is due to a liquid ejection event induced by meniscus pulsation. Again, due to the scale of the pulsation frequency (kHz), a naked eye or a slow camera is completely blind to these events. However, a fast camera can be triggered by the rising current, providing a trigger for fast time-lapse images.

The movies below may seem similar - they each show a pulsation cycle captured by triggering the camera on a specific current oscillation; however, each movie captures a sequence of a different electrospray regime.

Regime descriptionMovie
The dripping regime5 is the first regime an electrospray may experience at the lower range of applied voltages. During this regime, relatively large drops form at the emitter tip and fall due to a combined effect of gravitational and electrical forces. N/A
The burst regime6 (Axial I in the classification by Juraschek and Röllgen) is the second regime an electrospray may experience with increasing applied voltage. This regime is characterized by occasional rise and oscillation of the current followed by a period of inactivity, as reflected by the green waveform in the figure above. The movie corresponds to the highest intensity current peak within the burst.
The pulsating regime7,8 (Axial II in the classification by Juraschek and Röllgen) is the third regime an electrospray may experience with increasing applied voltage. This regime is characterized by continuous pulsation of the electrified meniscus, as reflected by the black waveform in the figure above. The movie is representative for any of the meniscus pulsations.
The astable regime9 is another regime an electrospray may experience with increasing applied voltage. In this regime, the electrospray spontaneously switches between the pulsating and the cone-jet modes. The movie corresponds to the first pulsation at the transition between the cone-jet to the pulsating mode.
The cone-jet regime10 received the most intense theoretical and experimental scrutiny. Many assume that the meniscus assumes this geometry when voltage is applied to a liquid pumped through a capillary. Blind selection of the operating parameters (as often the case) are very unlikely to lead to this regime. N/A

References:

  1. G Taylor Proc. R. Soc. London A 1964, 280: 383-397.
  2. L Rayleigh Philos. Mag. 1882, 14: 184-186.
  3. A Gomez, KQ Tang Phys. Fluids 1994, 6: 404–414.
  4. R Juraschek, FW Röllgen Int. J. Mass Spectrom. 1998, 177: 1-15.
  5. XG Zhang, OA Basaran J. Fluid Mech. 1996, 326: 239-263.
  6. I Marginean, P Nemes, A Vertes Phys. Rev. Lett. 2006, 97: 064502.
  7. I Marginean, L Parvin, L Heffernan, A Vertes Anal. Chem. 2004, 76: 4202-4207.
  8. I Marginean, P Nemes, L Parvin, A Vertes Appl. Phys. Lett. 2006, 89: 064104.
  9. I Marginean, P Nemes, A Vertes Phys. Rev. E 2007, 76: 026320.
  10. J Fernández de la Mora Annu. Rev. Fluid Mech. 2007, 39: 217-243.

Last update: January 31, 2009