AC Electrospraying

The group has pioneered the use of high-frequency (>100 KHz) AC fields to produce AC electrosprays of ethanol and other alcohols. In contrast to the classical DC electrospray, a steady Taylor cone from which drops emanate is not observed. Instead, within a narrow voltage window, drops are produced intermittently by a tip streaming, microjet ejection or drop pinch-off mechanism at the tip of the meniscus which resonates at a frequency associated with the capillary-inertia vibration time of the meniscus. In addition, the drops generated are approximately one to ten micron in diameter and hence are much larger than those observed in DC electrosprays. Moreover, unlike the drops produced by their DC counterpart which carry a net charge, these drops are electroneutral.

We believe that the distinct behaviour and attributes to that of DC electrosprays creates a wider spectrum of technological possibilities for electrospray applications. Whilst DC electrosprays have found applications in genomics and proteomics such as electrospray ionization mass spectrometry for protein and DNA characterization, we believe that these electrosprays, given its ability to generate electroneutral drops, will have commercialization potential in drug encapsulation and respiratory drug delivery systems such as inhalers for diabetic and asthma patients.We have initiated a collaboration with the Walter Cancer Research Center at Notre Dame on anti-cancer drug encapsulation and delivery.

By varying the applied voltage and frequency, several spray regimes have been categorized:

 

Tip streaming – In the limit of increasing viscous dominance, drops are ejected from the tip of a stable meniscus. In this mode, bi-periodicity is observed. The meniscus resonates at a low frequency f1 (~ 10 Hz) associated with the capillary-inertia vibration time scale of the drop and ejects multiple drops at a higher characteristic frequency f2 (~ 1000 Hz).

Microjet formation – At higher frequencies, intermittent microjets are formed at frequency f1 due to increasing inertial effects. This mode is bi-periodic (multiple drop ejection at frequency f2) at lower applied frequencies and decreases to single ejections with frequency f1 as the applied frequency is increased.

 

 

Wetting – Further increase of the applied frequency beyond a crossover frequency (see below), the polarization reverses resulting in an oppositely directed force towards the needle tip which pushes liquid up the needle as an apparent electrowetting effects. Drops pinch-off with frequency f1; however, as the wetting effects increase, drop pinch-off is suppressed. 

 

 

We attribute the mechanism of this AC electrospray to gas-phase polarization due to an plasma ionization reaction. The coions produced by such plasma reaction are dispersed for DC sprays or for low-frequency AC sprays. However, they are trapped neart the tip at much higher frequencies. In contrast, the relaxation time of the liquid charge separation is much longer and hence, at such high frequencies, polarization is absent on the liquid side. As a result, the high-frequency AC spray (>100 KHz) is indedpendent of the liquid conductivity and the ejected drops are uncharged. We are able to encapsulate drugs and carry out AC electrospinning of biofibers by spraying polymer solutions with this AC spray.

  References:

   Yeo, L.Y., Lastochkin, D., Wang, S.-C., and Chang, H.-C., ‘A New AC Electrospray Mechanism by Maxwell-Wagner Polarization and Capillary Resonance’, Phys. Rev. Lett., 92, 133902(2004).

Yeo,L. Y., Gagnon, Z. and Chang, H.-C., "AC Electrospray Biomaterial Synthesis", Biomaterials, 26, 6122 (2005).