Synergistic Morphology‐Material Design in a Hierarchical Composite Surface for High‐Efficiency Drag Reduction

While the riblet structures on shark skin are known to reduce frictional drag in turbulent flow, the synergistic contributions from the underlying cavity region and the flexible dermis remain inadequately explored, limiting the performance of existing biomimetic surfaces. To move beyond single-mechanism imitation, we present a biomimetic hierarchical composite surface (BHCS) featuring a normal elastic gradient that integrates denticle arrays, a flexible substrate, and interfacial chemistry. This integrated design is achieved through a multi-material 3D printing and spray-coating process. The optimized BHCS achieves a maximum drag reduction (DR) rate of 18.65% in underwater flow. Systematic experiments and numerical simulations reveal that this superior performance stems from a synergistic mechanism: the denticles induce stable longitudinal vortices that reorganize near-wall turbulence, while the flexible substrate enhances a unique reverse pore flow within the inter-denticle cavities through deformation and directs it against upstream structures. This impingement creates localized zones of elevated pressure, resulting in a net forward thrust that reduces the overall drag. Meanwhile, the surface-grafted PDMS molecular brush forms a liquid-like interfacial layer and increases surface hydrophobicity, thereby reducing interfacial shear. This research demonstrates a novel biomimetic design concept in which the synergy of multiple mechanisms leads to optimized DR.