Avalanche Coalescence of Liquid Metal Particles for Uniform Flexible and Stretchable Electrodes

Abstract

introduced to improve the coating quality by reducing the surface tension. During the LM ink fabrication, an inevitable oxide layer is spontaneously formed at the LM particle's interface, which serves as a barrier to electrification. Because of this, when the solvent of the LM ink is completely evaporated, either a sintering or an annealing process is mandatory to recover the electrical conductivity because the oxide skin of the LM particles is highly electrically insulating. [22] For these processes, typically, thermal energy (e.g., from a furnace, [23][24][25] a heater, [26,27] and a laser [28][29][30][31] ) and mechanical energy (e.g., tapping, [21,32] peeling off, [33] friction, [34] and compression [35] ) are applied.However, the additional sintering processes are inappropriate for the flexible electronics with soft materials. For the thermal sintering case, the temperature in the furnace is higher than 500 °C [23,24] definitely devastating the flexible materials. In the event of annealing, LM particles with hydrogen doping can be thermally annealed at 120 °C for 3 h, or LM particles can be chemically annealed on a substrate heated to 70 °C, which possibly makes damage on the flexible substrates with low glass transition temperature such as polyamide and polyethylene terephthalate. For the laser application, it could also destroy the soft substrate and alter the surface roughness due to locally intensified laser exposure. [36] In the case of mechanical sintering, the sintering process is very empirical and not quantitatively determined. [30] Furthermore, the physical contact with the LM deposition may cause damage to the printed LM pattern. Briefly, the conventional mandatory process, that is, either the sintering or annealing process, should be avoided for the versatile flexible printed electronics.Recently, some studies were introduced to avoid sintering post-processes, such as drying in vacuum [37] and adding nanoparticles including nanofibrils [38] and nanoclays. [39] Nonetheless, to realize the reliable, convenient, and low-cost fabrication of flexible printed electronics, the previous techniques are still far from complete because each method has inevitable and inherent bothersome aspects, for instance demanding drying step [37] and electrically insulated Janus structures by additive nanomaterials. [38,39] Thus, to achieve a conductive uniform and flexible electrode for soft robotics, wearable and flexible electronics, a simple and rapid coating method should be developed.In this study, we constructed an LM ink composition for flexible electrodes and introduced a drop-casting coating method Self-sinterable gallium-based liquid metal (LM) ink for a handy drop-casting coating method is developed. The fabricated LM electrode is coffee-ring-free and bilayer-free, which guarantees high electrical conductivity, wettability, flexibility, and stretchability. The ink consists of Galinstan, ethanol-water mixture, hydrochloric acid, and polyvinylpyrrolidone. The binary mixture provides excellent wetting condition...

Liquid Metal Micro- and Nanodroplets: Characteristics, Fabrication Techniques, and Applications

“…During solvent evaporation, the presence of PVP enhanced the steric forces between individual liquid metal droplets, which subsequently dried into a uniform layer. 10 Attaching low concentrations of well-defined diblock copolymers to the oxide skin can form polymer brushes that are highly effective for the stabilization of liquid metal nanodroplets. 11 Polymer brushes are especially useful in that blocks with multiple coordination sites can attach to the surface of liquid metal droplets.…”

“…On the other hand, polymeric/molecular surfactants or rheology additives (e.g., colloidal clays such as laponite) interact with the metallic surface to stabilize the droplets from coalescence during ultrasonication. 10,15 Ultrasonication generates a substantial amount of heat, which may act as a double-edged sword. The use of a highly volatile solvent or poor sealing can lead to evaporation of the liquid medium during sonication.…”

“…When there is an excess of water in the liquid medium, the surface coating of stabilizing polymers may be affected by the attraction between water and metal ions. , To resolve these issues, various methods have been established for the stabilization of liquid metal droplets in the emulsion state. In a recent study, researchers introduced hydrochloric acid and polyvinylpyrrolidone (PVP) to a water–ethanol binary mixture containing liquid metal droplets . While the acid partially removed the oxide membrane, PVP could effectively suppress the coalescence of liquid metal droplets dispersed in the mixture.…”

“…While the acid partially removed the oxide membrane, PVP could effectively suppress the coalescence of liquid metal droplets dispersed in the mixture. During solvent evaporation, the presence of PVP enhanced the steric forces between individual liquid metal droplets, which subsequently dried into a uniform layer …”

“…Identifying the effects of ambient oxygen solubility and electric fields (e.g., electrocapillarity, continuous electrowetting, electrowetting-on-dielectric, and electrochemically controlled capillarity) on liquid metal droplets provides insight into how the metallic surface interacts with its environment. The interface between a liquid metal droplet and its surroundings can be further modulated by applying various surface functionalization techniques. ,− When it is required to merge the droplets into continuous networks, we may introduce additional processes of thermal, laser, or mechanical sintering. Based on these characteristics, researchers have sought a number of different methods to tailor the design of liquid-metal droplets.…”