Objective Ice accretion and its subsequent removal can be great threats to aircrafts, power lines, wind turbines, marine structures, and even the pipes of air conditioners or refrigerators, which may lead to serious life safety problems and enormous economic loss. Traditional deicing methods, such as mechanical vibration deicing, electro-thermally deicing, or chemical fluid deicing are usually energy-intensive and/or environmentally unfavorable. Alternatively, emerging passive anti-icing (for prevention or delay of ice accumulation) and icephobic (for easy removal of ice) surfaces have been widely studied. Among them, superhydrophobic surfaces are promising candidates due to their extreme high-water repellency. However, superhydrophobic-based ice-resistant surfaces are facing three possible problems, including low humidity tolerance, relatively high ice adhesion strength which needs to be further reduced and poor deicing mechanical durability. In the present study, we report a novel kind of triple-scale micro/nano-structured superhydrophobic surface with comprehensive anti-icing and icephobic properties via ultrafast laser hybrid fabrication. This type of superhydrophobic surface exhibits excellent Cassie state stability, high humidity resistance, and good deicing durability. We hope that our basic strategy and findings can be helpful for the design of new robust ice-resistant superhydrophobic surfaces and the relationships between superhydrophobicity and ice resistance.
Methods Copper and aluminum alloys have been employed in the present study. First, the triple-scale micro/nano structures, composed of microcone arrays covered with densely grown nanograsses and dispersedly distributed micro and/or submicron flowers, were fabricated on the surfaces via a hybrid method combining ultrafast laser ablation and chemical oxidation. Then, the resultant surfaces were chemically modified by fluoride to induce superhydrophobicity. After that, contact angle and sliding angle of the surfaces were tested on a video-based optical contact angle measuring device. Then, the morphologies and chemical compositions of the textured surfaces were analyzed by scanning electron microscopy and X-ray diffraction. The effects of chemical oxidation time on the morphology and superhydrophobicity of the prepared surfaces were studied. In the next step, condensation observations and icing delay experiments were performed on the optimized superhydrophobic surfaces to assess their anti-icing performance. Furthermore, ice adhesion strength and icing-deicing cycles were also measured and performed for the prepared superhydrophobic surfaces to characterize their icephobic properties.
Results and Discussions The prepared triple-scale micro/nano-structured surface possesses excellent superhydrophobicity with a contact angle greater than 160° and a sliding angle less than 1° (Fig. 3). With increasing oxidation time, the nanostructures formed on the microcone arrays on the surfaces evolved from nanorods to nanograsses via hydrolysis (Figs. 4 and 5). Overall, the resultant contact angle increases and the sliding angle decreases with increasing oxidation time (Table 3). The anti-icing function study shows that the optimized superhydrophobic surface is featured with hierarchical condensation and coalescence-induced jumping of the condensed droplets under condensation and freezing conditions due to its low surface adhesion (Figs. 6 and 7). Since the air pockets trapped in the surface structures perform as a thermal barrier layer, the prepared superhydrophobic surface exhibits good icing delay performance with an icing delaying time of 52 min 39 s (Fig. 8). The icephobicity study of the prepared superhydrophobic surfaces shows that the ice adhesion strength of the superhydrophobic surface can be as low as 6 kPa, which is 40 times lower than that of the original aluminum alloy surface (Fig. 10). In addition, after 10 repeated icing-deicing cycles, the ice adhesion strength of the superhydrophobic surfaces are still no more than 20 kPa (Fig. 10), demonstrating decent deicing robustness.
Conclusions In the present study, a novel kind of triple-scale micro/nano-structured superhydrophobic surface, composed of periodical microcone arrays covered with densely grown nanograsses and dispersedly distributed micro/submicro-flowers, were successfully fabricated via ultrafast laser hybrid method. After chemical modification, such a surface possesses excellent superhydrophobicity with a contact angle greater than 160° and a sliding angle less than 1°. The surface morphology evolution shows that the superhydrophobicity of the prepared surface is determined by the surface roughness and hierarchical level. The observed hierarchical condensation phenomenon on the prepared superhydrophobic surface ensures the Cassie state stability of the primary condensed droplets even under high humidity and the condensed droplets can slide off the surface before freezing due to low surface adhesion, thus enabling the prepared superhydrophobic surface with great anti-icing performance. The ice adhesion strength of the superhydrophobic surface can be as low as 6 kPa, which is very competitive even compared with the interfacial slippage surfaces and the low interfacial toughness surfaces (the reported ice adhesion strength can be as low as 5 kPa), indicating that superhydrophobic-based icephobic surfaces can also exhibit ultralow ice adhesion. Our study shows that such kinds of triple-scale micro/nano-structured superhydrophobic surfaces with comprehensive anti-icing and icephobic properties can be obtained through rational surface design, which couples multi-scale micro/nano roughnesses and hierarchical levels.
