Ultrafine fibers produced by electrospinning often exhibit bead-on-string structures, which have generally been considered to be undesirable "by-products" or defects. Theoretical analysis in the literature predicted three types of instabilities for an electrically driven jet: the axisymmetric Rayleigh instability, the electric field-induced axisymmetric, and whipping instability. The process of bead formation revealed that the formation of a beaded structure resulted from axisymmetric deformation and flow of the jet. Applied voltage, solution surface tension, and conductivity (or jet charge density carried by the moving jet) were theoretically demonstrated to be important for jet axisymmetric instabilities. Experimental results revealed that these parameters influenced the formation of beaded fibers in the same manner as they did for the axisymmetric instabilities. As a result, the axisymmetric instabilities were considered to be the most likely mechanism of beaded fibers formation during electrospinning. POLYM. ENG. SCI., 45:704 -709, 2005.

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BACKGROUND: Some of the problems with electrospun zein fiber are that it has very low tenacities in the dry and wet states and that mats of the fiber become films when immersed in water. The fibers are therefore unusable for various applications despite their good biocompatibility and biodegradability. This research was conducted to overcome these problems by electrospinning novel fibers containing various concentrations of zein, citric acid (CA) and sodium hypophosphite monohydrate (SHP) and by crosslinking the zein with CA and with SHP serving as a catalyst. RESULTS:The CA-crosslinked electrospun zein fiber has as much as 10-fold greater wet tenacity and 15-fold greater dry tenacity than regular electrospun zein fiber. The average diameter of these fibers is 451 nm, which is the smallest diameter ever reported for zein-based electrospun fiber. A mat of this fiber retains its fibrous structure when immersed in water, and the fiber retains about 70% of its tenacity after 16 days at 50 • C and 90% relative humidity. CONCLUSION:The high dry and wet tenacities, good water stability and small diameter of the novel CAcrosslinked electrospun zein fiber make it attractive for biomedical and other applications that expose zein to water or that require high surface area.

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Self-aligned growth of ultra-short single-walled carbon nanotubes (SWNTs) was realized by utilizing optical near-field effects in a laser-assisted chemical vapor deposition (LCVD) process. By introducing the optical near-field effects, bridge structures containing single suspended SWNT channels were successfully fabricated through the LCVD process at a relatively low substrate temperature. Raman spectroscopy and I-V analyses have been carried out to characterize the SWNT-bridge structures. Numerical simulations using a high-frequency structure simulator revealed that significant enhancement of local heating occurs at metallic electrode tips under laser irradiation; it is about one order of magnitude higher than that in the rest of the electrodes. This technique suggests a novel approach to in situ low-temperature fabrication of SWNT-based devices in a precisely controlled manner, based on the nanoscale heating enhancement induced by the optical near-field effects.