Abstract
The air retention capability of a superhydrophobic surface plays the crucial role of drag reduction in an aqueous environment. Here, fabrication of water-repellent hybrid structural surfaces by synthesizing superhydrophobic nanowires with a high aspect ratio on micro-scale denticle structures to improve their air holding capacity in water is reported. The hybrid structure is realized by carrying out polymer molding of denticle structures on flexible substrates, hydrothermal growth of nanowires, and subsequent ultra-thin film coating. This technique is readily applicable to large areas, and the fabricated substrates are attachable onto curved surfaces. Our engineered, super water-repellent hybrid structures are found to effectively maintain air bubbles on their surfaces in a highly shear flow condition with a wall shear stress of up to 33.4 Pa, due to the combined effects of the micro-scale denticle structure, which reduces flow resistance, and the superhydrophobic, high-aspect-ratio nanowire structure, which enhances the capillary force to maintain the air bubbles. Our results show the importance of developing superhydrophobic structures of improved air retention capability.
Highlights
These findings show the importance of the capability of a structure to hold an air layer on its surface in a high Reynolds number flow or highly shear flow for frictional drag reduction[15]
We demonstrate the fabrication of superhydrophobic hybrid structures combining micro-scale denticle structures with high-aspect-ratio nanowires to effectively maintain air bubbles in a highly shear flow
The nanowires on flexible PDMS substrates with micro-scale denticle structures are coated with a polytetrafluoroethylene (PTFE) ultra-thin film
Summary
Developing nano- and micro-scale structures on the surfaces can reduce frictional drag without additional devices or energy consumption has been actively studied for various structures, such as ships, underwater vehicles, and piping systems[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23]. Drag reduction of up to 50% in the boundary layer flow of water in an open channel with a Re ranging of 1~2 × 105 was observed[15] Such reduction effects decreased above a Re of 2 × 105 and disappeared around a Re of 2.6 × 105, because the air bubbles on the surface broke away. These findings show the importance of the capability of a structure to hold an air layer on its surface in a high Reynolds number flow or highly shear flow for frictional drag reduction[15]. Our results show the promise of developing superhydrophobic hybrid surface structures with effective air retention
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