Revealing the Diffusion Coffee Ring on Hydrophilic Paper for Popular Science Learning of Liquid Surface Tension and Capillary Action in Microchannels

Understanding liquid surface tension and capillary action is vital for the study of fundamental chemical theories and advanced technologies, but these studies are still insufficient. It is essential to facilitate the public understanding of surface tension by designing a series of engaging and useful teaching experiments. Due to the liquid surface tension, aqueous solutions can spontaneously move along microchannels in a specific direction. To investigate this spontaneous flowing behavior, we fabricate coffee-loaded Xuan paper using those common household materials, coffee and Xuan paper, which reveals the fundamental theory and application principles of liquid surface tension. Moreover, an in-depth analysis of the capillary-action-driven migration and diffusion of coffee microparticles within those microchannels of Xuan papers is conducted. Significantly, the migration of coffee results in a visually striking phenomenon of diffusion coffee ring (DCR), which is regulated by factors such as particle sizes, paper hydrophilicity, and liquid types. Inspired by DCR, we designed a paper-based chemical sensor to monitor ethanol concentrations with features such as simple operation, low cost, and environmental friendliness. As our DCR protocols exhibit strong feasibility and high potential for at-home implementation, it can be adapted as multimode teaching experiments for different groups, from elementary-school students and middle-school students to undergraduates and even community residents. Our design indicates that the fundamental truth is often simple. It not only uses those common household materials to elucidate the core scientific concepts of liquid surface tension and capillary action but also promotes the integration of those simple life phenomena with advanced science and technologies, effectively facilitating the dissemination and studies of scientific knowledge.