Experimental Characterization of electrospinning: the electrically forced jet and instabilities
Original Entry by Holly McIlwee, AP225 Fall 09
Experimental Characterization of Electrospinning: the electrically forced jet and instabilities. Y. M. Shin, M. Hohman, M. P. Brenner and G. C. Rutledge, Polymer, 42,9955-9967(2001).
Electrospinning, Electrified Fluid Jet, Instability
Sub micron polymer fibers are formed from solution by subjecting the solution to a high electric field. The author has chosen to investigate the jet of polymer that is produced by this electric field and its instabilities.
Traditionally polymer fibers are formed using a technique called extrusion which applies pressure on a polymer melt to great a continuous fiber or profile. The extrusion process is not optimized for sub-micron features without many defects or mechanical instabilities. In order to form sub-micron fibers, the electrospinning process has been employed more recently. The attractiveness of these fibers stems from their high surface area to volume ratio as well as their nanoscale . The first reference to the electrospinning process was in 1934.  But more recently in 1995,  it was confirmed that submicron fibers were formed.
As of 2001 over 30 polymers had been electrospun. Material, equipment, and operating parameters can have a great effect on the resulting fibers and up until the time of this paper, many of these issues were not extensively studied. Until this time there has not been a quantitatively accurate theory, which Brenner et al. believes in a hinderance to comparing and analyzing experimental results. Brenner sets out to show that jet formation and instability can be characterized by using a small set of operating parameters.
Many soft matter considerations are relevant when utilizing electrospinning for fiber formation. Of particular importance: viscosity, conductivity, dielectric constant, and surface tension. Important operating parameters are: flow rate, jet current, applied electric potential, and distance between spinneret and collector.
It was found that the shape of the jet is dependent on the E-field and flow rate. When the electric field in increased the jet thins more rapidly and the Taylor cone becomes shorter and more concave. Beyond the Taylor cone, the jet thins more slowly. This review will highlight the jet instability analysis done by Brenner et al. Steady state jet profiles can be calculated and numerically compared to experiments. Two assumptions are necessary for this: the fiber is infinitely long, and it is a slender jet.
For example when using Glycerol, a material with low conductivity, a high E-field is necessary to obtain stable jets. This is achieved by using high voltage or small separation distance, although the latter makes measuring the jets difficult. It was found that Glycerol jets were formed when the needle protruded beyond the plate, in a parallel plate system. Taylor observed that there exists a critical voltage at which a jet or droplet appears. Increased protrusion increases the E-field on the polymer and therefore lowers the voltage necessary to yield a jet. The E-field changes shape here. This phenomena is called the 'fringe field'. Here the nozzle and top plate are both at the same potential.
Electrospinning and other methods of sub-micron fiber formation are very interesting to those in the soft matter field, not only for the physical phenomena, and yet unknown parameters and properties, but also from an application standpoint. As stated above, the high surface area to volume ratio offers a vast landscape for nanomanipulation and interaction.
 Formhals A. US Patent 1,975,504, 1934.  Doshi J, Reneker DH, J Electrostatics 1995; 35; 151-160.