Wettability alteration in thiolene-based polymer microfluidics: surface characterization and advanced fabrication techniques

Wettability plays a significant role in controlling multiphase flow in porous media for many industrial applications, including geologic carbon dioxide sequestration, enhanced oil recovery, and fuel cells. Microfluidics is a powerful tool to study the complexities of interfacial phenomena involved in multiphase flow in well-controlled geometries. Recently, the thiolene-based polymer called NOA81 emerged as an ideal material in the fabrication of microfluidic devices, since it combines the versatility of conventional soft photolithography with a wide range of achievable wettability conditions. Specifically, the wettability of NOA81 can be continuously tuned through exposure to high-energy UV. Despite its growing popularity, the exact physical and chemical mechanisms behind the wettability alteration have not been fully characterized. Here, we apply different characterization techniques, including contact angle measurements, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) to investigate the impact of high-energy UV on the chemical and physical properties of NOA81 surfaces. We find that high-energy UV exposure increases the oxygen-containing polar functional groups, which enhances the surface energy and hydrophilicity of NOA81. Additionally, our AFM measurements show that spin-coated NOA81 surfaces have a roughness less than a nanometer, which is further reduced after high-energy UV irradiation. Lastly, we advance the state-of-the-art of NOA81-based microfluidic systems by creating i) a 2D surface with controlled wettability gradient and ii) a 3D column packed with monodisperse NOA81 beads of controlled size and wettability.