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...
A drying multi-component liquid droplet in a confined geometry leaves a uniform dried pattern. The evaporated vapors are stagnated inside the closed chamber, which induce Marangoni effects that contribute to suppress the coffee-ring pattern.
Currently, quantum dot light‐emitting diodes (QD‐LEDs) are receiving extensive attention. To maximize their luminous performance, the uniformity of the QD‐LEDs is crucial. Although the spontaneously self‐induced solutal Marangoni flow of an evaporating binary mixture droplet has been widely investigated and used to suppress coffee‐ring patterns in ink‐jet printing technology, unfortunately, ring shapes are still present at the edges, and the Marangoni flow generated by the selective evaporation of volatile liquid components cannot be controlled due to its nonlinear instabilities. In this work, polygonal coffee‐ring‐less QD microarrays are created using two spontaneous and sequential solutal Marangoni flows. During the initial evaporation, internal circulating flows are controlled by polygonal‐shaped droplets. After that, sequential interfacial flows are generated by the captured volatile vapors. A theoretical model and scaling analysis are provided to explain the working mechanisms. It is expected that the newly designed printing system can be applied to the mass production of QD‐LEDs.
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