Enhanced air layer stability of hierarchical grooves with heterogeneous wettability surfaces

The air layer on the underwater vehicle effectively transforms solid–liquid contact into air–liquid contact, leading to a significant reduction in frictional resistance. However, the short reservation time and small volume of the air layer severely limit its drag reduction function. To address these challenges, this study proposed a novel approach combining hierarchical groove structures with heterogeneous wettability patterns to improve the stability of the air layer. First, the air layer morphology in the step-groove with different surficial wettabilities was investigated through experiments and simulations. Results showed that superhydrophobic step-grooves form stable macroscopic air layers through their superior air entrapment capability. Furthermore, the hierarchical groove structures were fabricated on the step-groove surfaces using laser processing. Experimental results demonstrated that the hierarchical groove structure maintained air layers more durably than single-structured step-grooves, primarily due to the enhanced air entrapment capacity of microgrooves and the improved structural robustness. Subsequently, by systematically investigating heterogeneous wettability patterns on the three internal walls of the step-groove, it was found that the pattern with superhydrophobic microgrooves surfaces on the left and upper walls, and an original surface on the right wall, exhibit an optimal performance of air layer reservation. This optimized step-groove structure can achieve air reservation rates of 88.3% and 81.9% at flow velocities of 1 and 2 m/s for 2 h, respectively. This structure demonstrates a capability to maintain a stable air layer underwater and thus can be utilized for underwater drag reduction in low-speed flows.