Needleless electrospinning. I. A comparison of cylinder and disk nozzles

In this study, we demonstrated the needleless electrospinning of poly(vinyl alcohol) (PVA) nanofibers with two nozzles, a rotating disk and a cylinder, and examined the effect of the nozzle shape on the electrospinning process and resultant fiber morphology. The disk nozzle needed a relatively low applied voltage to initiate fiber formation, and the fibers were mainly formed on the top disk edge. Also, the PVA concentration had little influence on the disk electrospinning process (up to 11 wt %). In comparison, the cylinder electrospinning showed a higher dependence on the applied voltage and polymer concentration. The fibers were initiated from the cylinder ends first and then from the entire cylinder surface only if the applied voltage were increased to a certain level. With the same polymer solution, the critical voltage needed to generate nanofibers from the disk nozzle was lower than that needed to generate nanofibers from the cylinder. Both electrospinning systems could produce uniform nanofibers, but the fibers produced from the disk nozzle were finer than those from the cylinder when the operating conditions were the same. A thin disk (8 cm in diameter and 2 mm thick) could produce nanofibers at a rate similar to that of a cylinder of the same diameter but 100 times wider (i.e., 20 cm long). Finite element analysis of electric field profiles of the nozzles revealed a concentrated electric field on the disk edge. For the cylinder nozzle, an uneven distribution of the electric field intensity profile along the nozzle surface was observed. The field lines were mainly concentrated on the cylinder ends, with a much lower electric field intensity formed in the middle surface area. At the same applied voltage, the electric field intensity on the disk edge was much higher than that on the cylinder end. These differences in the electric field intensity profiles could explain the differences in the fiber fineness and rate of the nanofibers produced from these two nozzles. These findings will benefit the design and further development of large-scale electrospinning systems for the mass production of nanofibers for advanced applications.

Parametric study of different fins for a rocket at supersonic flow

Various parameters including the fins with variable span to chord ratio, curvature radius, and setting angle have been investigated between the flat fin and wrap around fin (WAF) rocket configurations at supersonic flow. The results show that under the same flight condition, the flat fins can provide a higher lift and pitching moments than the WAFs. Due to the symmetric effect, any extra side forces, moments as well as the self-induced rolling characteristics will be not generated as compared to the WAF configurations. The WAFs can greatly improve the longitudinal stability and enhance the longitudinal aerodynamic characteristics for the whole rocket. The static pressure distributions at different chordwise positions together with the force variations around the fins have been obtained computationally and analyzed in detail.

Disk-electrospinning of PVA nanofibres

In this study, we demonstrated that a thin aluminium disk can be used as nozzle to electrospin PVA nanofibres on a large-scale. A schematic of this electrospinning system and a SEM image of as-spun PVA nanofibers are shown in Figure 1. The lower part of the disk is inside a bath containing the polymer solution, which is connected to a high voltage powder supply. During electrospinning, the disk rotates and picks up a thin layer of electrically charged PVA solution. A large number of fibres are then electrospun simultaneously from two sides of tile disk and deposited on the electrode collector.
With the small prototype unit we used, the fibre production rate can be as high as 6.0 which is about 270 times higher than that of a corresponding normal needle electrospinning system (0.022g/hr). The effects of appliedb voltage, the distance between the disk nozzle and collector, and PVA concentration on the fibre morphology were examined. The dependency of fibre diameter on the PVA concentration showed a similar trend to that for a conventional electrospinning system using a syringe needle nozzle, but the diameter distribution was wider for the disk electrospun fibres in this study.
The profiles of electric field strength in disk electrospinning showed considerable dependence on the disk thickness, with a thin disk exhibiting similar electric field profile to
that of a needle electrospinning system, but a thick disk (cylinder) exhibiting levelled electric field between the disk and the collector. PVA nanofibres electrospun from disk electrospinning were compared to that electrospun from syringe needle and metal cylinder nozzles.

Effect of nozzle polarity and connection on electrospinning of polyacrylonitrile nanofibers

In this study, two power supplies having positive/ground and negative/ground electrode output ends were used separately for electrospinning of polyacrylonitrile nanofibers. Depending on type of power supply and electrode connection, electrospinning led to different fiber diameters and deposition areas. The nozzle was connected to a high voltage end while the collector was grounded. Regardless of power supply used, finer fibers with a larger deposition area were obtained, compared to that using the same setup but with a reverse electrode connection. With an increase in the applied voltage, fiber deposition area, and productivity increased for all electrode connections. Grounded nozzles provide much better control over fiber deposition than the reverse electrode connections. Finite element modeling was used to analyze the electric field profile in the electrospinning zone. It was revealed that high electric intensity was mainly located in the part that was charged with a high voltage electrode, which could explain the differences in fiber diameter and deposition area.