Charge Separation in the Aerodynamic Breakup of Micrometer-Sized Water Droplets

Abstract

Charged water droplets generated by electrospray, sonic spray, and a vibrating orifice aerosol generator (VOAG) have been studied by digital macrophotography and image charge detection mass spectrometry. Image charge detection mass spectrometry provides information on the droplet size, charge, and velocity after transmission through a capillary interface. The digital images provide the droplet size distribution before they enter the capillary. Droplets with 10-100 microm radii generated by sonic spray and VOAG are reduced to 2-3 microm radii by transmission through the capillary interface. The droplets from sonic spray and VOAG are much more highly charged than expected for random charging, and positive droplets are much more prevalent than negative. For positive mode electrospray, >99% of the detected droplets carry a positive charge, whereas for negative mode electrospray, <30% of the detected droplets carry a negative charge (i.e., >70% carry a positive charge). These observation can all be accounted for by the aerodynamic breakup of the droplets in the capillary interface. This breakup reduces the droplets to a terminal size at which point further breakup does not occur. Charge separation during droplet breakup is responsible for the relatively high charges on the sonic spray and VOAG droplets and for the preference for positively charged droplets. The charge separation can be explained using the bag mechanism for droplet breakup and the electrical bilayer at the surface of water.

Mass Spectrometric Sampling of a Liquid Surface by Nanoliter Droplet Generation from Bursting Bubbles and Focused Acoustic Pulses: Application to Studies of Interfacial Chemistry

“…The transfer time of a droplet through the mass spectrometer inlet capillary is dependent upon the gas flow rate through the inlet, which can be approximated using the Poiseuille equation, 7,8 …”

“…While this approximation does not take into account the change in gas flow velocity across the transfer tube, the transfer time was also found to be in good agreement with previous studies and serves as an order of magnitude estimate for gas transfer time. 7,10,11 When the aspirated droplet initially enters the capillary, its velocity along the axis of the capillary is near zero, and the acceleration of the droplet to the carrier gas velocity requires a non-negligible amount of time. This rate of acceleration can be estimated from the drag force F D exerted on the droplet by the flowing gas, given by 12…”

Dry Deposition of Biogenic Terpenes via Cationic Oligomerization on Environmental Aqueous Surfaces

“…11,12 This charge separation process is variously labeled pneumatic or sonic spray ionization. 2,13,14 Such charged microdroplets subsequently evaporate solvent in the chamber while being drawn to the electrically polarized inlet of the mass spectrometer with increasing acceleration. Since these microdroplets are the progeny of nascent droplets stripped from the surface of the microjet, 8,9 the ES mass spectra acquired in these experiments therefore report the ion composition of the outermost layers of the initial stage of microjet just reacted with gaseous terpene.…”

Ion-Specific Long-Range Correlations on Interfacial Water Driven by Hydrogen Bond Fluctuations

“…As expected from the preceding analysis, in the case of neat water droplets, the finer droplets originating from the film are negatively charged because they contain excess anions (OH -), whereas the coarser ones arising from the rims carry net positive charge (H + ). These mechanisms were experimentally confirmed by Zilch et al 3 The enhanced surface and electrostatic energies of the polarized stretched films are drawn from the kinetic energy of the gas.…”

“…2 This observation is consistent with the droplet breakup mechanism shown above and previous reports by other researchers. 3 Under the present high v g (~230 m/s) condition, ions reside at topmost layers of the air-water interface are preferentially detected as mass signals. Further experimental details and validation tests could be found in elsewhere.…”