Improved Direct Ink Writing of Liquid Metal Foams via Liquid Additives

Abstract

Ga-based liquid metals (LMs) are conductive liquids in room conditions, making them attractive for printing conductive patterns. Yet, LM extrudes as droplets because of its high effective interfacial tension and low viscosity. In contrast, liquid metal foams (LMFs) exhibit yield stress and shear-thinning behavior, which are necessary for extruding filaments for direct ink writing (DIW). LMFs are made by stirring LM in air, thereby entraining oxide-lined air capsules. Unlike LM mixtures containing metallic particles, LMFs do not embrittle from intermetallic phase formation. Here, DIW of LMF and the challenge of separation of dissimilar phases during extrusion are explored. Incorporating additives, glycerol and tannic acid (TA), into LMF improves the printability of the mixture. The new formulations, LMFG (LMF + glycerol) and LMFGT (LMF + TA-glycerol solution), extrude more uniformly and produce better print quality than LMF. Interestingly, the viscosity and flow stress of LMF is between that of LMFGT and LMFG. This highlights the limitation of relying on rheological properties alone to predict printability in the case of LMF-based mixtures. With LM as the continuous phase, these LMF-based inks exhibit high electrical conductivity. DIW of LMF-based inks can be done in room conditions without additional pre/ post-processing, among other advantages.