Hydrochloric acid-impregnated paper for liquid metal microfluidics

We report a method of substantially improving lyophobicity of a paper against gallium-based liquid, using hydrochloric acid (HCl) impregnation. We also report an extremely simple fabrication of microfluidic channels on a HCl-impregnated paper for gallium-based liquid metal, Galinstan^®. Paper has many favorable properties in microfluidic applications, such as low cost, abundant supply, easy fabrication, and flexibility. We evaluated lyophobicity of several papers by measuring static and dynamic contact angles with a goal of finding optimal super-lyophobic paper-based substrate for microfluidics applications. We found HCl-impregnated laser printer flattened paper towel shows appropriate super-lyophobicity. We performed a bouncing experiment of a Galinstan^® droplet on the HCl-impregnated flattened paper substrate and demonstrated movement of a liquid metal droplet in microfluidic channels formed on the HCl-impregnated paper.