Spatially heterogeneous superwettability in groove surfaces: anchored plastron for underwater drag reduction

The air layer, known as a plastron, on superhydrophobic surfaces plays a crucial role in underwater drag reduction. However, stabilizing the plastron in complex turbulent flow conditions is a significant challenge, which impairs its effectiveness in drag reduction. This study proposes a streamwise groove structure with heterogeneous superwettability (HS-groove). The surface of the groove is treated through alkaline etching and water bath processes to form micro-needle-like crystals, resulting in superhydrophilicity with a water contact angle (CA) near 0° on the top surface. The interior of the groove is modified with hexadecyltrimethoxysilane (HDTMOS), imparting superhydrophobicity, with a CA exceeding 160° and a water roll-off angle (RA) of approximately 2°. This HS-groove structure demonstrates superior underwater plastron stability and drag reduction performance compared to traditional superhydrophobic grooves. Under continuous water flow, the plastron within grooves exhibited long-term persistence, with the maximum duration reaching 7.3 h and the corresponding drag reduction rates ranging from 17.2% to 46.4% in the pipeline. The strong surface energy barrier of the HS-groove enables effective plastron retention, leading to drag reduction performance superior to that of pure superhydrophobic grooves. This study offers novel insights and potential applications in underwater drag reduction technologies.