Coupled Bionic Drag-Reducing Surface Covered by Conical Protrusions and Elastic Layer Inspired from Pufferfish Skin

Inspired by the drag-reducing properties of the cone-like spines and elastic layer covering the pufferfish skin, important efforts are underway to establish rational multiple drag-reducing strategies for the development of new marine engineering materials. In the present work, a new drag-reducing surface (CPES) covered by conical protrusions (sparse "k-type" with rough height k+ = 13-15) and an elastic layer are constructed on copper substrate via a hybrid method, combining the sintering and coating processes. The drag-reducing feature of the prepared CPES biomimetic surface is achieved by rheometer and particle image velocimetry (PIV) experiments. To comprehensively investigate its drag reduction mechanism, the porous copper substrate (PCS), copper substrate (CS), conical protrusion resin substrate (CPRS), and conical protrusion porous copper substrate (CPPCS) were used for a comparative analysis. In laminar flow, we discovered that the conical protrusion structure and wettability of the elastic surface coupling affect the CPES sample's drag-reducing performance (7-8%) and that the interface produced slip to reduce the viscous drag. In turbulent flow, the CPES biomimetic surface exhibits an 11.5-17.5% drag-reducing performance. Such behavior was enabled by two concurrent mechanisms: (i) The conical protrusions as vortex generators enhance the number of vortices and the wake effect, enabling faster movement of downstream strips, reducing viscous drag; (ii) The conical protrusion elements break and lift large-scale vortices to produce numerous small-scale vortices with low energy, effectively weakening perturbations and momentum exchange. Additionally, the elastic layer shows high adhesion and stability on copper substrate after sandpaper abrasion and water-flow erosion tests. The copper substrate surface formed by the sintering method is also covered with dense porous structures, which gives the elastic layer and conical protrusions excellent combined robustness. Our findings not only shed new light on the design of robust drag-reducing surfaces but also provide new avenues for underwater drag reduction in the field of marine applications.