ELECTRONETTING

Nanomaterials and nanostructures, as critical factors in the recent advancement of many key tech nologies, have brought broad technological implications to areas ranging from environment, energy, electricity, national defense, and military projects to health care (Fang et al., 2011; Zhang and Fang, 2010; Chen and Shi, 2016; Yetisen et al., 2016). Nanomaterials differ from macrosized materials not only in the scale of their characteristic dimensions, but also in their capacity to provide signifificantly improved performance and new possibilities for various practical applications. One-dimensional (1D) nanoscale materials, such as nanofifibers, nanotubes, nanowires, and nanorods, are attracting dramatically increasing interest due to their unique electronic, thermal, mechanical, optical, and chemical properties (Lu et al., 2011; Xia et al., 2003; Yuan et al., 2011). Among these 1D nanomaterials, the nanofifibers have gained much attention due to their fascinating properties, for example, small pore size, large specifific surface area, high porosity, and ease of functionalization (Zhang et al., 2016a; Goh et al., 2013). Taking their robust advantages into account, various strategies have been created to fabricate nanofifiber materials. Compared with methods such as drawing (Li et al., 2013a), phase separation (Mao et al., 2012), template synthesis (Qiu and Mao, 2010), and seaeisland spinning (Zhang et al., 2015b), electrohydrodynamics techniques, especially electrospinning, stand out as the mainstream of largescale nanofifiber construction technology, in view of their ability to produce nanofifibers with controlled dimensions, structures, and functional components (Ahmed et al., 2015; Sun et al., 2014; Zhang et al., 2016c). Nanofifibers with complex architectures, including fifibers with porous structures, coreeshell structures, hollow structures, and helical structures, have been successfully fabricated from organic, inorganic, and hybrid materials (Wang et al., 2013a, 2017; Si et al., 2014, 2016). And, the resultant nanofifiber aggregates show varied structures, like random orientation, alignment, patterning, and 2D networks (Wang et al., 2013b; Chen et al., 2014; Ding et al., 2010).

Electronetting, as a cutting-edge electrohydrodynamics technique, was developed based on the electrospinning process (Ding et al., 2006; Zhang et al., 2015a; Hu et al., 2011; Yang et al., 2011). Both these techniques involve the process of creating nanofifibers via an electrostatic force between a Taylorcone of precursor solution and a receiving collector. By virtue of a suffificiently high voltage electrostatic fifield, the electrical force overcomes the surface tension of the charged solution, and then enables the charged flfluids (e.g., jets and droplets) to be ejected from the Taylor cone. Then, the charged flfluids solidify with rapid solvent evaporation as they travel in air, and deposit on the collector to assemble into various nanofifiber materials. However, compared with conventional electrospinning, there are some unique characters of the electronetting strategy (Zhang et al., 2015a). In addition to the 1D common nanofifibers (the same as the electropsun nanofifibers), 2D nanowire networks with Steiner tree structures (namely nanonets) form synchronously, meaning that the electronetting technique possesses the capacity for a one-step preparation of 3D nanofifiber/net materials composed of traditional nanofifibers and 2D nanowire networks (Wang et al., 2010a, 2012c, 2013b; Nirmala et al., 2010a). In this 3D nanofifiber/net material, the 2D fifishnet-like or spiderweb-like networks consisting of interlinked ultrafifine nanowires are supported by using electrospun nanofifibers as the scaffold. Benefifiting from the combined structural features, the resultant 3D nanofifiber/net materials possess not only the fundamental properties and functions derived from 1D conventional nanofifibers, but also some fascinating features, including extremely small diameter and Steiner tree pore structure, among others. By virtue of these intriguing characters, and its large specifific surface area, small pore size, and high porosity, this nanofifiber material exhibits great potential for various applications, ranging from sensors and air fifiltration to individual protection (Wang et al., 2011a, 2012a, 2012b; Li et al., 2013b; Nirmala et al., 2014).

Obviously, the 3D nanofifiber/net materials have evoked wide attention as novel ultrafifine nanomaterials, which scientists are exploring in a wide range of applications. Many kinds of nanofifiber/net membranes involving polyamide (PA), polyvinyl alcohol (PVA), polyacrylic acid (PAA), polyurethane (PU), and gelatin have been prepared, and several possible mechanisms have been proposed to clarify their formation process (Wang et al., 2013b; Zhang et al., 2015a; Kimmer et al., 2009; Parajuli et al., 2009; Barakat et al., 2009). Moreover, signifificant progress has been made in the application of the nanofifiber/net materials, for example, in sensor applications, air fifiltration, and tissue engineering. In this chapter, the emerging electronetting technique and its resultant nanofifiber/net materials are systematically introduced from their origin and defifinition, formation mechanism, and structural regulation to their applications. Moreover, we carefully highlight the recent advances in the controllable fabrication, properties, and performance of nanofifiber/net membranes in specifific applications. We conclude with a summary of current research efforts and prospects for future research in the development of electronetting and nanofifiber/net materials.

(A) Schematic illustration of a horizontal electrospinning/netting apparatus setup. (B) The forces acting on the charged droplet. (C) A typical fifield emission scanning electron microscopy image of a nanofifiber/net membrane.

Paper link：https://www.sciencedirect.com/book/9780323512701/electrospinning-nanofabrication-and-applications