Inkjet printing is a promising technique for large-scale printed flexible and stretchable electronics. However, inkjet printing of silver nanowires (AgNWs) still presents many challenges. In this study, inkjet printing of high-concentration AgNW ink on flexible substrates is demonstrated, and liquid polydimethylsiloxane (PDMS) is then spin-coated on top of the printed AgNWs patterns to form stretchable conductors. We analyzed the relationship between the surface microstructure and electrical property of the stretchable AgNW conductors during various stretching/releasing cycles. Three consecutive stages of the resistance change, including conditioning, equilibrium, and rising stages, can be observed as a result of the morphology change. We also have demonstrated that the inkjet-printed stretchable AgNW conductor can be used as a stretchable heater. All of these characteristics indicate the excellent potential of inkjet printing of AgNWs for developing large-area flexible and stretchable electronics.

interaction, ultimately determine the final deposition and morphology of patterns. [4][5][6][7][8][9] Various approaches have been explored to modify these interactions in order to control the contact line dynamics and/or to induce Marangoni flow driven by the surface tension gradient on the air-droplet interface, including tuning substrate wettability, [10,11] application of surfactant additives and cosolvent systems to the droplet, [12,13] vapor absorption of low-surface tension solvents to the droplet, [14] etc. In these techniques, the colloidal particles are carried back to the center of the droplet either by the depinned contact line or by Marangoni flow to suppress the coffeering effect. In addition, stronger interactions between colloidal particles and substrates, e.g., electrostatic and van der Waals interactions, [15] and increased adhesion through substrate treatment, [16] etc. also facilitate a uniform deposition. Recently, attempts have been made to push the colloidal particles onto the droplet surface to facilitate the particle selfassembly at the air-droplet interface. [17] Boley et al. have employed a cosolvent system for the colloidal particles. [18] During evaporation, the colloidal particles which are well-dispersed in the solvent with a higher vapor pressure are carried to the droplet surface due to faster evaporation of this solvent component. Li et al. have accelerated the solvent evaporation rate to trap the particles at the droplet interface by elevating the environment temperature. [19] At a high environment temperature, the air-droplet interface shrinkage rate exceeds the particle diffusion rate such that the colloidal particles are captured by the descending surface, producing a particle jam which prevents them from being transported to the droplet edge. The charges of surfactant-decorated particles have also been tuned to become nearly neutral, [20,21] which promotes particle trapping at the air-droplet interface to render a homogeneous deposition.In this work, we employ a dual-droplet configuration to transform the Langmuir-Blodgett (LB) concept to the picoliter droplets generated by inkjet printing. Deposition of monolayer nanoparticle films is achieved by a consecutive dual-droplet printing of a supporting droplet and a wetting droplet (Figure 1). The supporting droplet acts as the LB trough, and the wetting droplet contains colloidal nanoparticles. The colloidal particles spread over the supporting droplet surface and assemble at the interface as the solvent dries to produce a uniform, nearlyThe well-known coffee-ring effect causes colloidal particles to convectively transport toward the contact line of an inkjet droplet leading to a nonuniform deposition of the colloidal particles. In this work, the self-assembly of colloidal particles in a dual-droplet inkjet printing configuration to produce a nearly monolayer closely packed deposition of colloidal particles that exhibits a colorful reflection are demonstrated. By controlling the ink surface tensions and jetting parameters, the ...

The evaporation of particle-laden droplets on a substrate usually results in ring-like deposits due to particle migration to the contact lines. This ubiquitous phenomenon, known as the coffee-ring effect (CRE), was initially observed in drying coffee droplets and later in many colloidal systems. The CRE has been intensively investigated during the past two decades to unveil the complexity related to its flow patterns, evaporation physics, and deposition structures after solvent evaporation. However, the contribution of colloidal particle assembly and interactions at the air–liquid interface of sessile droplets to the particle deposition requires more attention. The objective of this Review Article is to highlight the recent advances in mitigating or totally suppressing the CRE by means of interfacial assembly via manipulating the multibody interactions, for example, particle–particle, particle–substrate, particle–flow, and particle–interface interactions. Well-ordered monolayer deposition of the colloidal particles, driven by interfacial assembly, has been demonstrated by several research groups. This unique perspective of suppressing the CRE and creating well-ordered monolayer structures by assembling colloidal particles at the air–liquid interface creates a new paradigm in generating coatings and functional devices through liquid processing. General rules and guidelines are established to provide broader prospects of engineering desirable structures of colloidal particle deposition and assembly.

Stretchable conductors have attracted tremendous attention owing to their potential applications as electrodes and sensing elements for wearable electronic devices. In this study, we have developed an inkjet printing process to directly embed and align silver nanowires in the polydimethylsiloxane elastomer for stretchable conductors. Instead of printing on top of elastomers, we printed the silver nanowires directly into an uncured liquid elastomer layer to embed the conductive nanowires, therefore eliminating the problems with surface wetting and delamination in conventional approaches, where the conductor is patterned on top of elastomers. This study investigated the process controls to tune the embedment depth and alignment of silver nanowires through inkjet printing. The printing process was captured with a high-speed camera to elucidate the mechanisms that direct the printed features. By controlling the thickness of the liquid elastomer layer and the post-printing treatment of printed lines, stretchable conductors composed of embedded silver nanowires have been fabricated in a single step. Because of the reflow of the viscous liquid elastomer, the printed silver nanowires are aligned along the printing direction, resulting in linewidth of tens of micrometers upon solvent removal. The stretchable conductors showed good mechanical and electrical stability under repeated bending and stretching/releasing cycles. This method of directly embedding silver nanowires into a liquid elastomer offers facile heterogeneous integration for digital patterning of stretchable electronics.

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