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Reducing resistance to surface friction is challenging in the field of engineering. Natural biological systems have evolved unique functional surfaces or special physiological functions to adapt to their complex environments over centuries. Among these biological wonders, fish, one of the oldest in the vertebrate group, have garnered attention due to their exceptional fluid dynamics capabilities. Fish skin has inspired innovation in reducing surface friction due to its unique structures and material properties. Herein, drawing inspiration from the unique properties of fish scales, a periodic array of fish scales was fabricated by laser ablation on a polished aluminum template. The morphology of the biomimetic fish scale surface was characterized using scanning electron microscopy and a white-light interfering profilometer. Drag reduction performance was measured in a closed circulating water tunnel. The maximum drag reduction was 10.26% at a Reynolds number of 39,532, and the drag reduction performance gradually decreased with an increase in the distance between fish scales. The mechanism of the biomimetic drag reduction surface was analyzed using computational fluid dynamics. Streamwise vortices were generated at the valley of the biomimetic fish scale, replacing sliding friction with rolling friction. These results are expected to provide a foundation for in-depth analysis of the hydrodynamic performance of fish and serve as new inspiration for drag reduction and antifouling. [ABSTRACT FROM AUTHOR]

The integration of creatures and nature has become more and more perfect during biological evolution. Unique structure characteristics, material properties, and special functions of organisms can provide an inexhaustible source for human scientific thoughts, technological inventions, and solutions to engineering and technical problems. The high-speed swimming capacity of aquatic organisms has long attracted growing attention in recent years. Underwater drag reduction is a functional strategy formed by the long-term evolution of high-speed swimming organisms such as fish. Revealing the connection between the structure characteristics, mechanical properties of fish skin and drag reduction provides a feasible solution to the high surface frictional resistance. This paper reviews the topography characteristics and material properties, bionic fabricating, and drag reduction mechanism of fish skin. In the end, both existing problems and future research prospects of bionic surfaces are proposed. This paper may provide a better and more comprehensive understanding of the current research status of fish skin inspired drag reduction and provide a feasible reference for further revealing the excellent hydrodynamic performance of fish and solving the surface friction resistance on high-speed moving surfaces. • Fish skin has different unique structural characteristics and material properties • Bionic surfaces inspired by fish skin have shown excellent drag reduction performance • The coupling surface with unique structure and functional coating will have much more progress in the years ahead [ABSTRACT FROM AUTHOR]

Fish skin has a unique biological feature that hard fish scales are embedded in a flexible dermis layer and covered by a viscoelasticity mucus or compliant epidermis layer. Therefore, the entire fish skin system can act as a dynamic resilient energy-absorbing coating with micro-roughness to absorb external turbulent pulsations. The microscopic morphology, arrangement of fish scales, and mechanical characteristics of tuna skin inspired the extraction and fabrication of the bionic gradient flexible fish skin (BGFFS) model. First, BGFFS was fabricated by the three-dimensional (3D) printing method, spraying and casting process. Then, the 3D morphology of the bionic fish skin was characterized. The results indicated that the BGFFS exhibited good morphology of fish scales and spatial gradient mechanical properties like fish skin. Moreover, the experimental results in a circulating water tunnel showed that the BGFFS had excellent drag reduction performance in a turbulent flow. When the Reynolds (Re) number was 6.8 × 104, the maximum drag reduction ratio could reach 13.8 %. The drag reduction mechanism was analyzed by the computational fluid dynamics (CFD) method, and the energy-absorbing effect of the BGFFS was measured by the polyvinylidene fluoride (PVDF) piezoelectric film. In this experiment, the maximum voltage signal reached 0.448 V. The drag reduction mechanism of the BGFFS was that the rolling friction caused by vortexes replaced sliding friction. In the meantime, the BGFFS could absorb the micro disturbances in a turbulent flow. These findings can serve as a foundation for an in-depth analysis of the hydrodynamic performance of fish as well as a new inspiration for drag reduction and antifouling. • The entire fish skin system of fish skin can act as a dynamic resilient energy-absorbing coating with micro-roughness • Maximum drag reduction ratio could reach 13.8% when the Reynolds (Re) number was 6.8×104 • The mechanism of drag reduction of the bionic gradient flexible fish skin was rolling friction and absorb the micro disturbances in a turbulent flow. [ABSTRACT FROM AUTHOR]