Paper-Based Microfluidics: Fabrication Technique and Dynamics of Capillary-Driven Surface Flow

Paper-based devices provide an alternative technology for simple, low-cost, portable, and disposable diagnostic tools for many applications, including clinical diagnosis, food quality control, and environmental monitoring. In this study we report a two-step fabrication process for creating two-dimensional microfluidic channels to move liquids on a hydrophobized paper surface. A highly hydrophobic surface was created on paper by TiO₂ nanoparticle coating using a high-speed, roll-to-roll liquid flame spray technique. The hydrophilic pattern was then generated by UV irradiation through a photomask utilizing the photocatalytic property of TiO₂. The flow dynamics of five model liquids with differing surface tensions 48–72 mN·m^–1 and viscosities 1–15 mN·m^–2 was studied. The results show that the liquid front (l) in a channel advances in time (t) according to the power law l = Zt^0.5 (Z is an empirical constant which depend on the liquid properties and channel dimensions). The flow dynamics of the liquids with low viscosity show a dependence on the channel width and the droplet volume, while the flow of liquids with high viscosity is mainly controlled by the viscous forces.