Hydrochloric acid-impregnated paper for gallium-based liquid metal microfluidics

We report a simple hydrochloric acid (HCl) impregnation method to substantially improve the lyophobicity of a paper against gallium-based liquid metal. Based on the HCl-impregnated paper, we also propose an extremely simple fabrication method of microfluidic channel for gallium-based liquid metal, Galinstan®. Due to its low cost, easy fabrication, and flexibility, recently paper has drawn attention as microfluidic platforms for various applications. We have treated two different types of paper (paper towel and printing paper) with various treatment methods such as laser printer flattening, fluorocarbon polymer coating, HCl-impregnation, and combination of these methods. We then studied their lyophobicity characteristics by measuring static and dynamic contact angles as well as bouncing experiment. We found that HCl-impregnation is a simple yet powerful method to engineer certain types of papers to make them super-lyophobic substrates against gallium-based liquid metals and effective for more than 30-days after impregnation. To show the feasibility, we demonstrated manipulation of a Galinstan® droplet along microfluidic channels formed on the HCl-impregnated paper.