Robust Cassie State of Wetting in Transparent Superhydrophobic Coatings

Abstract

This paper investigates the stability of the Cassie state of wetting in transparent superhydrophobic coatings by comparing a single-layer microporous coating with a doublelayer micro/nanoporous coating. Increasing pressure resistance of superhydrophobic coatings is of interest for practical use because high external pressures may be exerted on surfaces during operation. The Cassie state stability against the external pressure of coatings was investigated by squeezing droplets sitting on surfaces with a hydrophobic plate. Droplets on the single-layer coating transformed to the Wenzel state and pinned to the surface after squeezing, whereas droplets on the double-layer micro/nanoporous coating preserved the Cassie state and rolled off the surface easily. In addition, the contact angle and contact-line diameter of water droplets during evaporation from surfaces were in situ investigated to further understand the stability of coatings against Wenzel transition. A droplet on a microporous coating gradually transformed to the Wenzel state and lost its spherical shape as the droplet volume decreased (i.e., the internal pressure of the droplet increased). The contact line of the droplet during evaporation remained almost unchanged. In contrast, a water droplet on a double-layer surface preserved its spherical shape even at the last stages of the evaporation process, where pressure differences as high as a few thousand pascals were generated. For this case, the droplet contact line retracted during evaporation and the droplet recovered the initial water contact angle. The demonstrated method for the preparation of robust transparent superhydrophobic coatings is promising for outdoor applications such as self-cleaning cover glasses for solar cells and nonwetting windows. KEYWORDS: Cassie state stability, superhydrophobic, self-cleaning, wettability, evaporation, organically modified silica ■ INTRODUCTIONA water droplet sitting on a superhydrophobic surface (i.e., the water contact angle is >150°) can be in either the Wenzel or Cassie state depending on the morphology and chemistry of the surface. In the Wenzel state, the surface is completely wetted and the droplet pins to the surface, whereas in the Cassie state, the droplet wets the surface partially and air pockets form between the surface and water droplet. 1−3 The Cassie state of wetting provides a large apparent water contact angle with low contact-angle hysteresis and results in a roll-off superhydrophobic surface, which is desired for practical applications such as self-cleaning windows and solar panels, 4−6 underwater drag reduction, 7,8 and anticorrosion coatings. 9 Various successful methods have been developed by mimicking well-known examples from nature (e.g., Lotus leaves and butterfly wings) to prepare artificial superhydrophobic surfaces with the Cassie state of wetting using lithographic methods, 10−12 sol−gel techniques, 4,9,13−15 phase separation in polymer blends, 16 electrospinning, 17,18 and others. 5,19−23 However, the Cassie state of ...

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