Tribological metamaterial: how feathers reduce drag and friction through hidden energy dissipation structures

The lateral moving resistance of a liquid droplet on a solid surface generally increases with velocity and is dominated by the non-viscous wetting line friction. Many superhydrophobic man-made and biological surfaces have minimal, nevertheless speed-sensitive, water droplet friction, limiting their potential to reduce drag at high speeds in natural situations. Using an in situ surface force apparatus, we demonstrated low and remarkably speed-insensitive (over 300-fold) water bridge sliding friction on a goose feather vane. Detailed analyses suggest a dominant, hidden energy dissipation channel probably related to the deformation and elastic recovery of feather's characteristic metamaterial-like structure, which also results in feather's speed insensitive (from 0.1 to 1 mm s^-1) ultra-low dry sliding friction coefficient observed in this study (approx. 0.07). The new insights gained have the potential to motivate novel approaches to the design of all-weather and speed-insensitive low-friction surfaces with practical applications in aviation and lubrication technology.