Stability of Air Bubbles and Air Layers on Superhydrophobic-hydrophobic Surfaces and Their Drag Reduction Performance

YANG Qianfan; SHI Xuesong; ZHANG Tongbin; XU Xuefeng

doi:10.16490/j.cnki.issn.1001-3660.2026.04.013

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Surface Technology ›› 2026, Vol. 55 ›› Issue (4) : 160-170. DOI: 10.16490/j.cnki.issn.1001-3660.2026.04.013

Functional Surfaces and Technology

Stability of Air Bubbles and Air Layers on Superhydrophobic-hydrophobic Surfaces and Their Drag Reduction Performance

YANG Qianfan^1,2,3, SHI Xuesong^1,2,3,*, ZHANG Tongbin^1,2,3, XU Xuefeng^1,2,3

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Abstract

Superhydrophobic surfaces have great potential in reducing the frictional drag of underwater vehicles. Uniform superhydrophobic surfaces can trap air underwater and form an air layer, but the stability and morphologies of liquid-air interfaces on superhydrophobic-hydrophobic surfaces and the corresponding drag reduction performance still remains unknown. This work aims to fabricate superhydrophobic-hydrophobic surfaces to investigate the stability of the liquid-air interfaces and examine the drag reduction performance. Closed microcavities were fabricated on the surface of 20 mm × 80 mm × 5 mm 7075 aluminum alloy plates by adjusting the pulse energy, scanning speed and path of a violet nanosecond laser. The specific areas of different widths (0.5, 1.0, 1.5, 2.0, 2.5 mm) were reserved to avoid being fabricated by laser, by which different types of superhydrophobic-hydrophobic surfaces were obtained after fluorination treatment. The contact angles of the superhydrophobic region and the hydrophobic region were 158.8° and 131.2°, respectively. The sliding angle of the superhydrophobic region was 0.6°. Air layer observation and drag reduction evaluation were performed with a home-built circulating water tunnel facility, which included a water tank, a circulating pipe, a pump, a test section, and two valves. This device could simulate underwater navigation conditions by using flowing liquid that completely immersed the sample to be tested. When the flow rate was in the range of 1.53-4.43 m/s, both air layers and air bubbles could be formed on the superhydrophobic-hydrophobic surface, whereas only an air layer could be formed on the uniform superhydrophobic surface. The air bubbles existed in the form of strips, namely "air bubble strips", which were located at the junction of the superhydrophobic region and the hydrophobic region. These air bubble strips had a significant drag reduction effect under certain flowing conditions. At the lower flow rate (1.53 m/s), the maximum drag reduction rate was 35.56% when the width of hydrophobic region was 0.5 mm, which was higher than that of the uniform superhydrophobic surface. With the increase of water flowing time, the coverage of air bubbles and air layers gradually decreased and finally disappeared. The air bubbles could sustainably maintain for ~12 min whereas the air layer could maintain for ~6 h. The average drag reduction rate of the superhydrophobic-hydrophobic surface was higher than that of the uniform superhydrophobic surface within 0.5 h. At the higher flow rate (4.43 m/s), the average drag reduction rate of the superhydrophobic-hydrophobic surface was lower than that of the uniform superhydrophobic surface. As a result, the drag reduction effect of the superhydrophobic-hydrophobic surface was better than the uniform superhydrophobic surface within 0.5 h when the flow rate was relatively low. Hence, at lower flow velocities and within a short period of time, due to the bubble strip, the drag reduction effect of the superhydrophobic-hydrophobic interfacial surface was superior to that of the uniform superhydrophobic surface. This study can provide significant guidance for enhancing the drag reduction rate of underwater vehicles, developing sustainable drag reduction strategies, and meeting the drag reduction requirements in their diverse application fields.

Key words

drag reduction / superhydrophobic-hydrophobic surface / superhydrophobic surface / air bubble / air layer

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YANG Qianfan, SHI Xuesong, ZHANG Tongbin, XU Xuefeng. Stability of Air Bubbles and Air Layers on Superhydrophobic-hydrophobic Surfaces and Their Drag Reduction Performance[J]. Surface Technology. 2026, 55(4): 160-170

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