Hydrophobic and Water-Repellent Modification of Polymeric Surfaces with Co-Curing of Silica Aerogel

Hydrophobic and water-repellent surfaces can be used in various applications including microfluidics and self-cleaning solar cell surfaces. Numerous studies reported enhanced hydrophobicity by creating micro-/nano-structures and/or spin-/spray-coating of hydrophobic materials. Conventionally, those approaches require complex fabrication processes such as photolithography, growth, etching, chemical synthesis, electrospraying, and so on. On the other hand, the aerogel is a porous nanostructure which is mostly filled with a gas. Due to its surface roughness, silica aerogels are one of attractive materials for hydrophobicity. Here, we report a simple method to create hydrophobic and water-repellent surface on various elastomers.<br/>We implemented hydrophobic surfaces on three different elastomers: Polydimethylsiloxane (PDMS; Dow Corning, USA), Dragon Skin 10 (Hyup Shin, Seoul, South Korea), and EcoFlex 5 (Hyup Shin). Those precursors were mixed with their corresponding curing agent in ratios recommended by each manufacturer (10:1, 1:1, and 1:1 for PDMS, Dragon Skin, and EcoFlex, respectively). Next, we spread a commercial silica aerogel powder (AP-1000, Gelhouse, Daejeon, South Korea) on the surface of each elastomer to cover the whole area. This simple step formed a thin layer that introduces hydrophobicity and water repellency. After this, samples were cured for 2 hours at 80°C in a convection oven. Finally, the aerogel powder which was not fixed onto the surface of the elastomers was removed by a blower.<br/>Structure and composition of the silica aerogel layer were analyzed by scanning electron microscope as well as energy-dispersive X-ray spectroscopy (EDX; Teneo VS, FEI). The silica particle size was measured to be ~20 µm in average. The EDX analysis showed Si and O are only components of aerogel. Next, to characterize their hydrophobicity, we placed a 20 µL of deionized (DI) water droplet on each elastomer surface. Contact angles (CAs) of PDMS, Dragon Skin, and EcoFlex were enhanced from 107°, 110°, and 99° to 134°, 132°, and 138° without and with silica aerogel co-curing, respectively; they were 25.2, 20.0, and 39.4% improvements in CAs. In addition to the hydrophobicity, the silica layer substantially boosted water repellency of each polymer surface. All samples demonstrated tilting angles &lt; ~5° which were remarkably reduced from their initial tilting angles (58°, 38°, and 36° for PDMS, Dragon Skin, Ecoflelx, respectively). To test the reliability of the fabricated hydrophobic layers, we immersed our samples in an ultrasonic bath (CPXH 3800, Branson Bransonic) at 110 W and 40 kHz for 5 min. The CAs of the samples were changed by &lt;10% from the original values.<br/>In this study, we introduced a simple and effective way to create a hydrophobic layer on the surface of various elastomers by co-curing silica aerogel powder. Both hydrophobicity and water-repellency of the three polymers were considerably increased. We expect our proposed method can be applicable to other kinds of elastomer to enhance their hydrophobicity for diverse applications.<br/><br/>* Hyeonhee Roh and Hyunsun Song both contributed equally to this work.