Surface Modification of 3D Biomimetic Shark Denticle Structures for Drag Reduction

Shark skin features superhydrophilic and riblet-textured denticles that provide drag reduction, antifouling, and mechanical protection. The artificial riblet structures exhibit drag reduction capabilities in turbulent flow. However, the effects of the surface wettability of shark denticles and the cavity region underneath the denticle crown on drag reduction remain insufficiently explored. Here, 3D printing is utilized to fabricate realistic staggered and overlapped denticle arrays, modified to achieve superhydrophilic, superhydrophobic, and hybrid configurations, including external riblets hydrophilic/internal cavities hydrophobic (ELIB), and vice versa (EBIL). Denticles of varying heights are also fabricated. The results indicate that superhydrophobic, ELIB, and EBIL denticles outperform superhydrophilic ones in reducing drag, achieving a peak drag reduction rate of ≈20%. Notably, shorter denticles further improve drag reduction. Reduced vortex formation within the underneath cavity correlates with improved drag reduction. These vortices can function similarly to rolling bearings while facilitating momentum exchange and increasing skin friction drag. Superhydrophobic or partially superhydrophobic denticles (ELIBD/EBILD) mitigate this effect. This study suggests that sharks may secrete mucus on specific sections of their denticles to further reduce vorticity and drag, offering novel insights into the biomimetic design of shark denticles for optimized drag reduction.