Assembly of Multi‐Stranded Nanofiber Threads through AC Electrospinning

Electrospinning refers to the formation of micrometer and sub-micrometer fibers under the application of a strong electric field to a polymer fluid, solution, or melt. Compared to other methods of fiber preparation like drawing, template synthesis, extrusion, phase-separation of polymeric mixtures, self-assembly of individual molecules, etc., electrospinning provides a robust method to form long fibers with a wide range of fiber diameters that can vary from nanometers to micrometers. Additionally, the basic setup for electrospinning is relatively uncomplicated and can be applied to a variety of polymer–solvent systems. These favorable attributes have led to the widespread study of electrospinning for applications in different fields including nanocomposites, biocatalysts, fabrics and protective clothing, optical sensing, drug delivery, wound dressing, tissue culture, filtration, etc. In synchrony with the study of electrospinning for many applications, there has also been significant fundamental interest in understanding the mechanism behind this process. Previous work on DC electrospraying/electrospinning has shown that under the application of an electric field, the meniscus deforms into a conical shape formed due to a balance between capillary and coulombic forces, and a liquid jet ejects out from the conical tip, which thins as it accelerates downstream. After traversing a small distance in a straight-line path, the electrospun jet then undergoes a non-axisymmetric whipping instability that increases the effective jet path significantly and causes the jet to stretch and become extremely thin, leading to the formation of polymeric fibers with sub-micrometer dimension. This increase in path length due to whipping instability is responsible for the formation of ultrathin fibers with electrospinning. This instability is also behind the appearance of the characteristic conical whipping cloud that is just the trace of a single fiber jet oscillating very rapidly. The whipping instability makes the accurate simulation of the jet path difficult, however, certain features like the operating regime for electrospinning as well as the appearance of the conical cloud have been predicted with accuracy. The fibers formed via this technique predominantly form a mesh-like structure with a random orientation of fibers, hence there has been considerable effort directed toward aligning fibers and controlling their orientation and placement. In contrast to the detailed study of DC electrospinning, there have been very few reports on electrospinning using an AC electric field; the few studies available have focused on either DC-superimposed AC fields, or AC studies of electrospraying/ electrospinning at a fixed frequency. Our preceding work on AC electrospraying had revealed a multitude of novel physical mechanisms, predominantly dependent upon the frequency time scale, suggesting that AC electrospinning might be quite different from DC electrospinning. In this communication, we present our observations on the AC spinning phenomena and its dependence on the frequency time scale. We generate multistranded threads exhibiting a completely different morphology. Besides simplifying the characterization of nanofibers, this new fiber pattern and its strong frequency dependence can significantly extend the applicability of electrospinning for specific applications. We proceed by describing our observations, followed by an attempt to explain the mechanism behind this behavior. The experimental setup consisted of a needle and a flat electrode connected to a high voltage AC system, a schematic is shown in Figure 1a. The fact that the spinning behavior under an AC field was different was obvious when a visible thread was seen emerging downstream from the needle instead of the mesh-like morphology seen with DC spinning. This thread could be more than a meter long and did not display any significant attraction toward the ground electrode, and consequently it could be easily deflected away. Once the thread descended below the ground electrode, it became totally free of any coulombic attraction from the electrode system, and could be easily deflected manually or even by weak air currents around the experimental setup. This eliminated the problems associated with fiber alignment and collection that is usually seen with DC spinning. This is experimentally demonstrated in Figure 1b, where a large thread could be manually woven onto a commercial coaxial cable. Mechanical spooling with a disk and rotor assembly should therefore be quite straightforward, suggesting that this technique can be easily scaled up for applications. Scanning electron microscopy (SEM) images, shown in Figure 2, revealed some very distinct features of these threads. They were actually a combination of multiple strands that were not only weaved or superimposed over each other, but were also, in fact, a continuous network with the strands fused together. The thread thickness was of the order of 10mm while the individual strands were approximately 100 nm thick, showing a large variation in size between them. Depending upon the applied potential and frequency, there could be 100 strands or more weaved to form a