AC Electrospraying & Electrospinning


Electrospraying or electrohydrodynamic atomisation using high frequency (>10 kHz) electric fields exhibit significantly distinct behaviour compared to DC or low frequency AC electrosprays. For moderately high conductivity fluids, the characteristic sharp conical meniscus, known as the Taylor cone, in DC electrosprays is not observed. Instead, the meniscus resonates vigorously at a frequency corresponding to the inverse of the capillary-inertia time scale, periodically ejecting droplets via a tip streaming mechanism or from the tip of a long slender microjet. Due to the high frequency of polarity reversal, there is insufficient time for charge separation and subsequently double layer tangential conduction to occur within the meniscus. As such, there is no charge accumulation at the meniscus tip to produce the focussing mechanism for the formation of a sharp conical shape. In addition, a consequence of this is that the 1-10 μm order ejected drops do not carry net charge. These electroneutral drops, which are of the correct size for maximum dosage, is therefore suitable for pulmonary drug delivery, wherein charge drops which lead to surface adsorption and compound ionisation effects are undesirable. The absence of charge also renders the current and hence power requirement for the device low, which then makes the device suitable for miniaturisation.

By using a biodegradable polymeric excipient dissolved in a suitable solvent as the working fluid, it is also possible to produce 1-10 μm polymeric microparticles. We have also demonstrated the possibility of encapsulating water in these microcapsules using the electrospray. It is also possible to synthesize fibers made of the same polymeric material. The figures show a 10 μm thick compound fiber, a mesh or network of 1 μm thick fibers; the pore size of the mesh is adjustable. By encapsulating growth factors into the fibers, it is then possible to generate biodegradable scaffolds for tissue and orthopaedic engineering. The SEM images below also show the possibility of forming ordered 100 nm order polymeric crystalline structures.

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

  2. 2.LY Yeo, Z Gagnon, H-C Chang. AC Electrospray Biomaterials Synthesis. Biomaterials 26, 6122 (2005) (PDF).

  3. 3.LY Yeo, JR Friend. Electrospinning Carbon Nanotube Polymer Composite Nanofibers. J Exp Nanosci 1, 177 (2006) (PDF).

Next: Electrowetting