Studies of the dimensional effects of pillar arrays on hydrophobicity and wettability improvement of aluminum surfaces

Jianlong Gao

Superhydrophobic and superhydrophilic surfaces have many practical applications in industry and scientific research area. The superhydrophobic surfaces are finding applications in a wide range of areas, e.g. water-repellent coatings, self-cleaning fabrics, and anti-icing films. They also hold great potential in the fields of microelectromechanical systems (MEMS) and microfluidics. By contrast, superhydrophilic surfaces have also been investigated for a variety of applications. For example, in air conditioning applications, the wettability of water condensed onto the aluminum surface could be critically important for the amount of condensed liquid remaining on the dissipation unit and the pressure drop across the heat exchanger. Polydimethylsiloxane (PDMS) and SU-8 (negative photoresist) are polymers commonly used in microfabrication technology. They could be applied to create superhydrophobic coatings. In our work, the design and fabrication processes of superhydrophobic surfaces are reported. Without further coatings or treatment, the PDMS structures can achieve contact angles close to 170° while the SU-8 pillar arrays can only achieve contact angle up to 140°. Additional attention is paid to the dimensional effect of the surface micro pattern on surface hydrophobicity. Both theoretical and experimental results demonstrate that as spacing between pillars increases, PDMS patterns show a clear transition phase from Cassie state to Wenzel state. The critical transition spacing of different pillar diameters show a close match with theoretical results. By comparison, SU-8 shows a continuous consistent change in surface wettability. Aluminum is a common material in industry and has been widely used in air conditioning industry due to its high-performance thermal (heat dissipation) property. In this work, the effectiveness of a simple and rapid method for wettability improvement - boiling water treatment was investigated. The wetting and drainage performance on aluminum surface samples were examined. The contact angles were observed to decrease from the original value of 94.5° to approximately 10° after treatment, and the reduction is correlated with the treatment length. Scanning electron microscopy and 3D optical profiler images suggest that the treatment facilitates oxidation of the sample surface, which greatly alters the micro-structures on the surface. These are believed to be the two main reasons of the droplet wetting behavior changes.

Studies of the dimensional effects of pillar arrays on hydrophobicity and wettability improvement of aluminum surfaces

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