(44b) Electrospinning Under An Ac Field : Synthesis Of Complex Nano-Fibers And Membranes

Electrospinning has gained widespread attention over the past few years for producing micro and nano-fibers for a wide range of applications like tissue engineering, bio-encapsulation, controlled release, surface coating of catalytic and filtration materials for fuel cells, etc. Its major advantages include a simple and inexpensive setup, the potential to design specific templates via patterned microelectrodes and its ability to generate fibers in the nanometer range (with large surface area per unit volume) that are not possible with conventional methods like casting or chemical vapor deposition. In most cases, a DC (direct current) electric field has been used for electrospinning. Here we discuss the use of an AC (alternating current) electric field for electrospinning, which brings in additional tunable parameters and complications to this process. Firstly the charge in the fiber alternates with time and secondly a new time-scale, corresponding to the period of the applied field is introduced in the problem. Furthermore it is known that there is a critical relation between the conductivity of the spraying sample and the applied frequency which determines the meniscus shape and spraying mode for AC electrospraying (S. Maheshwari and H.-C. Chang, "Anomalous conical menisci under an ac field-departure from the dc Taylor cone," Applied Physics Letters, 2006). A combination of these factors results in a variety of spinning behavior producing fiber patterns and morphologies distinctly different from DC electrospinning.

At low frequencies (<1 kHz) the behavior shows some similarity with DC spinning in production of fine fibers. However for the AC case there is a considerable entanglement of the fibers due to attraction between fiber segments of different charge. Instead of always being directed away from the spinning needle, the fibers get alternately attracted and repelled from the needle electrode. This oscillatory force on the spun fibers causes them to loop around themselves during their flight from the needle to the collector, which provides a method to control the extent of entanglement. There can be as many as 7 fibers coming out of a single junction and they can spontaneously assemble into a network structure without any weaving effort. For AC spinning, the surface coverage and fiber generation decreases with increase in applied frequency. There is also a dramatic variation in the fiber morphology with the applied voltage and frequency, with the occurrence of fibers with or without beads. At comparatively higher voltages, bead formation is reduced with uniformly smooth fibers being produced, as the higher Maxwell force elicits an elastic pressure from the stretched polymers to prevent capillary drainage into beads. Varying the time of flight and applied frequency can also be tuned for distinct fiber morphologies. At higher frequencies (> 40 kHz), the behavior becomes dramatically different. The whipping instability, which is prevalent in DC spinning, is negligible. Here the combination of applied frequency and liquid properties become important. We find that the variation of conductivity for PEO (Poly-ethylene oxide) mixtures determines the presence or absence of spinning, unlike the low frequency case. This is probably due to the variation in the meniscus shape with conductivity. For low conductivities at such frequencies, a uniformly growing cone is seen, with a very sharp conical meniscus. This can reduce the net flow rate from the meniscus and evaporation of the solvent around the meniscus can cause it to harden. With increasing conductivity, the meniscus forms micro-jets which can replenish the solvent supply quickly at the needle tip and support the spinning process. This frequency regime also generates micro-particles, which have been used for encapsulation (L. Y. Yeo, D. Lastochkin, S. C. Wang and H.-C. Chang, "A new ac electrospray mechanism by Maxwell-Wagner polarization and capillary resonance," Physical Review Letters, 2004 & L. Y. Yeo, Z. Gagnon and H.-C. Chang, ?AC electrospray biomaterials synthesis? Biomaterials, 2005). We summarize these unique features and applications of AC electrospinning in this talk.