Generation mechanism and suppression method of landing error of two successively deposited metal droplets caused by coalescence and solidification

摘要

Accurate deposition of metal droplets is crucial to limit geometrical defects in metal droplet-based 3D printing. However, unpredictable movements of the droplets displaced from the original landing location due to surface tension-driven flow before it is hindered by solidification, named landing error, lead to broken traces, irregular waved surfaces, or collapsed structures. In the present work, the effect of solidification at the early-time coalescence of two deposited metal droplets on the landing error was revealed. High-speed images were presented to observe coalescence dynamics. A volume of fluid (VOF) based numerical model was developed to analyze the generation mechanism of the landing error. An interesting phenomenon is observed that the second droplet absorbs more mass from the first droplet to form a bigger spherical crown with the increase of substrate temperature, resulting in the distinct final shapes. Inspired by this, a mass conservation model was proposed to predict the dependence of the landing error on substrate temperature. Metal droplets overlapping experiments were further conducted to verify the model and establish the relationship between the landing error and the substrate temperature. Predictions agreed well with experimental results. Finally, a strategy of increasing the thermal conductivity of the substrate to suppress the landing error at high substrate temperatures was proposed, which significantly expanded the processing window of the accurate deposition. This work can provide useful theoretical and experimental guidance for accurate printing of high-quality lines or patterns by utilizing metal droplets.