Periodically undulating air–liquid interface on a patterned superhydrophobic surface for drag reduction

Superhydrophobic surfaces have been widely applied to underwater drag reduction due to their capability of maintaining an air layer, which converts the solid–liquid interface into an air–liquid slip boundary. However, continuous air layers with millimeter-scale thickness are highly unstable and exist only temporarily due to the shear of the liquid flow. Here, a periodically undulating air–liquid interface was formed on a triangular-patterned superhydrophobic aluminum-alloy surface (TSS) fabricated by nanosecond-laser direct writing and hydrophobic treatment. The superhydrophobic and hydrophobic regions exhibited advancing contact angles of approximately 166° and 121°, respectively, while the rolling angle of the superhydrophobic region was approximately 0.8°. This wettability contrast provides a capillary pinning barrier that resists bubble displacement at the three-phase contact line, thereby promoting the formation of a periodically undulating air–liquid interface. The equal-area geometrical analysis identified a favorable full apex-angle range of 45°–65°, within which the equivalent chord length remained close to its maximum, providing favorable geometrical conditions for interfacial confinement. Experimentally, the triangular pattern with a 64° apex angle exhibited comparatively stable bubble morphology and air-layer retention, which is consistent with the theoretically favorable apex-angle range of 45°–65°. At a flow velocity of 2.0 m/s, the 64° TSS maintained an interconnected, periodically undulating air–liquid interface for up to 600 min under continuous water flow, demonstrating long-term air-layer retention. At 3.0 m/s, corresponding to a Reynolds number of 7.2 × 104, the air-layer coverage ratio remained at 25.14% after 90 min. In separate drag measurements under the corresponding air-layer states, the TSS exhibited a measured drag-reduction rate of 14.57% at 0.5 m/s. At 4.0 m/s, the measured drag-reduction rate reached a maximum of 25.71% and subsequently decreased to 9.86% as the retained air layer was progressively depleted. The results demonstrate that rational design of the triangular-patterned superhydrophobic surface can provide long-term air-layer retention together with appreciable underwater drag-reduction performance over the tested flow conditions.