Fabrication of Multiphase Liquid Metal Composites Containing Gas and Solid Fillers: From Pastes to Foams

Abstract

Gallium-based liquid metals (LMs) are suitable for many potential applications due to their unique combination of metallic and liquid properties. However, due to their high surface tension and low viscosity, LMs are challenging to apply to substrates in useful shapes, such as dots, wires, and films. These issues are mitigated by mixing the LMs in air with other materials, such as mixing with solid particles to form LM solid pastes or mixing with gases to form LM foams. Underlying these deceivingly simple mixing processes are complex and highly intertwined microscale mechanisms. Air microbubbles are inevitably incorporated while making LM pastes, making them partly foams. On the other hand, for foaming of the LM to occur, a critical volume content of solid particles must be internalized first. Consequently, both LM pastes and foams are multiphase composites containing solid and fluid microcomponents. Here, we systematically study the impact of the mixing procedure, solid particle size, and volume fraction (SiO2) on the air content of the multiphase LM composites. We demonstrate that decreasing the particle size and increasing their volume fraction substantially decrease the composite density (i.e., increases air entrapment). The foaming process can also be enhanced with the use of high-speed mechanical mixing, although leading to the formation of a more disordered internal structure. In contrast, manual mixing with larger microparticles can promote the formation of more paste-like composites with minimal air content. We explain the microscopic mechanisms underlying these trends by correlating macroscopic measurements with cross-sectional electron microscopy of the internal structure.