  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.
