Reversible Wetting Transition of Water from Hydrophilic to Superhydrophilic State with UV-Ozone- and Ar-Plasma-Exposed Nanoporous Alumina Membranes: Microcooling, Sensing, and Filtering Applications

The hydrophilic-superhydrophilic transition dynamics of water on the multifunctional nanoporous anodic alumina (NAA) membranes of various pore lengths (0.03-5 μm) fabricated by the acid anodization process is demonstrated. The original pristine alumina surfaces were found to be in the hydrophilic Wenzel state. The pristine NAA sample surfaces were modified to a superhydrophilic state upon UV-ozone (UVO) exposure for 1 min. The sample surfaces were also modified to the near-superhydrophilic state by Argon plasma (Ar-P) treatment for 1 min. Carboxylate ions incorporated inside the NAA matrix during the anodization process were found to play an important role in modifying the sample surfaces to be superhydrophilic. It was revealed from XPS analysis that the increment in the oxygen percentage and reduction in the carbon percentage were the key points behind the superhydrophilic state after UVO and Ar-P treatment. The NAA matrix was made functional as a nanofluidic system consisting of water after UVO and Ar plasma exposure which can be used for micro-cooling, sensing, and filtering applications. Reversible switching to hydrophilic state was found, leaving the sample surfaces to ambient after UVO and Ar-P exposure.