Electrical Bending and Mechanical Buckling Instabilities in Electrospinning Jets
The determination of the behavior of the jet path in the vicinity of the onset of the primary electrical bending instability is important for the orderly collection of the nanofibers produced by electrospinning. A stable jet was observed with a high frame rate, short exposure time video camera. The collection process was complicated but predictable within limits, so the design and creation of some two or three dimensional structures of nanofibers is feasible, if the considerations described below are incorporated into the design and production processes. The fluid jet in the straight segment of the path, and the more solid nanofibers in the coils of the primary electrical bending instability were collected on stationary and moving surfaces. The diameter and characteristic path of the jet depended on the exact distance between the orifice and the collector, if other parameters were not changed. The moving surfaces caused the various coils that were collected to be displaced rather than superimposed. The fiber collected on the moving surfaces preserved a record of the electrical and mechanical instabilities that occurred. If the straight segment was very fluid, the jet formed a series of small sessile drops on the collector, but when the jet was more solid, buckling occurred and produced small, complicated loops close to point at which the jet hit the surface. Buckling was observed during collection of the straight segment and the first coils of the electrically driven electrical bending instability. A moving inclined collector was used to collect the fibers. Surface velocities were up to about 5 meters per second. These velocities are commensurate with the velocities at which the solidifying jet approached the surface. A variety of structures of loops, both conglutinated and not, associated with the instabilities were created. The jets used in this work were formed from solutions of polyethylene oxide, nylon-6, poly lactic acid, and other polymers. Several solvents were used for some of the polymers, and details of the jet path changed when the solvent or the concentration changed. The jets issued from a pendent drop on a glass capillary with an orifice diameter of about 160 microns. A potential difference in the range of 500 to 13,000 volts was applied between the orifice and the collector. The distance from the orifice to the grounded collector varied from 1 mm to 30 cm. Interference colors associated with jet diameters around 10 microns were observed in the straight segment. The color patterns were stable, indicating that the process variations were small.