Fabrication of titanium-based microstructured surfaces and study on their superhydrophobic stability

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Based on the classical wetting theories, two theoretically predicted formulas of the apparent contact angles on square-pillar-array microstructured surfaces for Wenzel mode and Cassie mode have been derived, respectively. The theories of superhydrophobic stability on microstructured surfaces have been summarized. Four square-pillar-array samples were fabricated on titanium substrates by using the femtosecond laser micromachining technology, and wettability was analyzed by both experimental and analytical methods. The results showed that the titanium-based surfaces are superhydrophobic. The maximal apparent contact angle is up to 156.9°, while the corresponding sliding angle is 4.7°. Testing of the superhydrophobic stability of the surfaces showed that the maximal deviation of the apparent contact angles is only 0.6°. Analyses indicate that the stable superhydrophobicity of the supplied titanium-based surfaces is within a certain range and not perfect. To improve that, a practical controllable method is proposed herein for the design of a stable superhydrophobic surface.

I. INTRODUCTION

In general, a superhydrophobic surface refers to a surface with a water contact angle greater than 150° and a sliding angle smaller than 10°.1–3 Such surfaces are expected to be widely used in numerous industries as well as in people’s daily lives in the future.4 So far, materials for achieving superhydrophobicity by constructing microstructures on their surfaces have mainly been attached to inorganic nonmetallic materials and polymeric materials. In contrast, obtaining superhydrophobicity by microstructuring on metal surfaces has been the subject of extensive research. The former research was mainly focused on Al,5,6 Au,7–10 Ag,11 Cu,12–15 ZnO,16 etc. In 1998, Shibuichi et al.5 fabricated an anodically oxidized aluminum plate by anodic oxidation, which had a water contact angle of about 160° after being silanized on its surface and displayed superhydrophobicity; in 2005, C.H. Wang et al.9 prepared gold films with highly stable gold micro-nanostructures by using a galvanic exchange reaction under different conditions and obtained superhydrophobic surfaces with a water contact angle of about 165° after modification of the gold films. In 2006, S.T. Wang et al.12 successfully developed a one-step solution-

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Address all correspondence to this author. e-mail: [email protected] or [email protected] DOI: 10.1557/JMR.2008.0307 J. Mater. Res., Vol. 23, No. 9, Sep 2008

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immersion process by a morphology-formation technology based on biomineralization, and they constructed very stable superhydrophobic films on copper plates with water contact angles of about 162° and sliding angles less than 5°. These experimental results would be of great significance for both self-cleaning and antisepsis of metal. Although there are some studies about microstructuring on the surfaces of titanium and its alloys, most of them are focused on the mechanism of microstructure formation and the effects of pro