This chapter deals with the mechanisms and electrohydrodynamic modeling of the physical processes resulting in electrospinning of nanofibers with cross-sectional diameters approximately in the range 100 nm to 1 µm. These involve the physical nature of fluids used in electrospinning, leaky dielectrics, discussed in Section 5.2, and the formation of the precursor of electrospun jets, the Taylor cone, described in Section 5.3. Polymer jets in electrospinning possess an initial straight section, which is discussed in Section 5.4. Experimental observations of the key element of the electrospinning process, the electrically driven bending instability, which is similar to the aerodynamically driven jet bending of Chapters 3 and 4, are covered in Section 5.5. Section 5.6 describes the theory of the bending instability in electrospinning. Multiple jet interaction in electrospinning and needleless electrospinning are discussed in Section 5.7. Co-electrospinning and emulsion electrospinning of core–shell fibers (Section 5.8) are based on similar physical principles to electrospinning of monolithic nanofibers. The electrostatic field-assisted assembly techniques developed with the aim of positioning and aligning individual nanofibers in arrays and ropes are discussed in Section 5.9. Melt electrospinning of polymer fibers is briefly outlined in Section 5.10.

Electrospinning of polymer solutions

Electrospinning of polymer solutions, liquid crystals, suspensions of solid particles and emulsions employs an electric field of the strength about 1 kV cm−1. The first US patent on electrospinning was issued to Formhals (1934), but interest in this process was dormant until electrified jets of polymer solutions and melts were investigated as routes to the manufacture of polymer nanofibers (Baumgarten 1971, Larrondo and Manley 1981a–c, Doshi and Reneker 1995, Reneker and Chun 1996). In electrospinning, the electric force results in an electrically charged jet flowing out from a pendant or sessile droplet (see Figure 5.1). After the jet flows away from the droplet in a nearly straight line, it bends into a complex path and other changes in shape occur, during which electrical forces stretch and thin it by very large ratios, quite similar to the effects of the aerodynamic forces in melt- and solution blowing discussed in Chapter 4. After the solvent evaporates, solidified nanofibers are left.