Fish-Inspired Nanostructured Lubricated Surfaces Enhance Aquatic Locomotion

Abstract

Slippery, mucus-coated body surfaces are widespread in aquatic vertebrates, but their contribution to locomotion is unclear. Fish mucus has long been proposed to reduce drag, yet direct experimental evidence of a swimming benefit is limited. A series of four mucus-inspired, lubricant-infused aluminum substrates incorporating SLIPS (slippery liquid-infused porous surfaces) were fabricated to assess the effects of a slippery coating on motion under both static (drag-based) and dynamic (propulsive) conditions. The aluminum substrates were textured by conversion of the surface to nanostructured aluminum oxyhydroxide (boehmite), chemically functionalized, and infused with synthetic Newtonian liquids spanning the viscosity range of measured trout mucus and compared to that of unstructured and lubricant-free controls. Viscosity measurements were also performed on skin mucus collected from brook trout (S. fontinalis). SLIPS-coated surfaces were found to reduce the static drag by up to 9.1% and enhance the lift-to-drag ratio by up to 8.7%. Furthermore, SLIPS coatings improved both the thrust and efficiency of flapping plates actuated in heave and pitch by up to 3.6% and 4.1%, respectively, with performance improvements observed over a broad range of motion parameters, lubricant chemistries, and viscosities. Boundary layer flow measurements over a flat plate revealed that SLIPS experienced a reduced skin friction drag of up to 5.2% under laminar flow conditions. Nanostructured, functionalized, lubricated coatings thus act both to reduce drag and to enhance the propulsive thrust and efficiency of fishlike propulsion, providing one explanation for the presence of such structures on a diversity of animal surfaces.