CENTRIFUGAL FORCE

Using PEO as a model polymer, Peterson et al. produced nanofibers at a production rate of 6.5 mg/h cm2(Fig. 7.19). Important parameters that affect the nanofiber production rate include voltage, spinneret rotation speed, solution feed rate, distance between spinning head and collector, and solution concentration. In this way, polyacrylonitrile fibers were produced successfully. This technique is also compatible with various collectors, such as a moving belt for collecting large membranes, parallel collector electrodes for aligned fibers, and parallel collector electrodes with one rotating electrode for producing short yarns.

A SUMMARY OF NEEDLE-LESS ELECTROSPINNING SPINNERETS

Moreover, asymmetrical spinnerets, such as a coil (Fig. 7.18), could also produce uniform nano- fibers and nanofiber mats. Unlike the symmetrical spinnerets, the coil spinneret shows an uneven electric field distribution.

Disk

A slowly rotating disk, which is partially immersed in polymer solution, eliminates the need for self replenishment of the solution at the fiber-spinning edge (Fig. 7.15). It also ensures a fresh coating of solution on the disk surface and prevents the solution from solidifying at the disk edge during electrospinning. Rotating cylinder and disk spinnerets have been analyzed by Niu et al. The rim radius of a cylinder spinneret can reduce discrepancies in electric field intensity and influence the fiber productivity. Thinner disk spinnerets increase the electric field intensity, leading to finer nanofibers and higher throughput.

WIRE SPINNERETS

During the electrospinning process, the high voltage was applied to the stationary wires and a polymer solution was loaded on the surface of the wires by moving the wire through the polymer solution. Then, numerous jets were generated from the wire surface. It has a better spinning performance than the previous version, which use a rotating drum as spinneret.

CLEFT SPINNERET

Lukas et al. reported an electrospinning setup that used linear clefts as the spinneret (Fig. 7.6). The authors also developed a one-dimensional electrohydrodynamic theory to explain the process of electrospinning conductive liquids from an open plane surface (Fig. 7.6). During the electrospinning process, the amplitude of characteristic wavelength grew faster because of the electrical force. The fastest-growing stationary wave marked the onset of electrospinning from a free liquid surface. This theory not only predicated the critical value of the electrospinning process but also explained the upward needle-less electrospinning.

MAGNETIC FLUID

In 2004, Yarin and Zussman reported a needle-less electrospinning system that used a magnetic field to initiate the jet formation. The setup comprised a bottom layer of ferro magnetic fluid and an upper layer of polymer solution (Fig. 7.4A). When an external magnetic field was applied to the fluid system and an electric field was added simultaneously to the polymer solution layer, the ferromagnetic fluid triggered the formation of steady vertical spikes, which perturbed at the interlayer interface. Under the action of a strong electric field, these spikes were drawn into fine solution jets (Fig. 7.4B). Compared with multineedle electrospinning, this needle-less setup can produce polyethylene oxide (PEO) nanofibers with 12 times higher productivity. Nevertheless, the nanofibers electrospun are relatively coarser with larger diameter distribution.

Needle-less Electrospinning

Needle-less electrospinning appeared in the early 1970s, when Simm and coworkers filed a patent on using an annular electrode to electrostatically spin fibers for filtration appli cations. Next, Lukas et al. investigated the self-organization of charged jets initiated from the open free-liquid surface in the electrospinning process. Lin and colleagues developed a rotating spiral coil spinneret, which had a high fiber production rate with well-controlled fiber morphology. Liu et al. electrospun nanofibers by blowing air into the polymer solution. The bubbles generated assisted in jet initiation. Since 2008, growing research has been devoted to needle-less electrospinning. Research publications have been increasing constantly over the years. Over 100 articles about needle-less electrospinning or free-surface electrospinning have been published since 2007. The publication number between 2014 and 2016 was approximately 10 times more than that of 2007e2013 (Fig. 7.2A). Research is widespread in many countries. China, Australia, and the Czech Republic take about 80% of publications, followed by the United States, Germany, and England (Fig. 7.2B).