Controlled wrinkling patterns and surface friction by thickness gradient

Gradient changing patterns often endow the surfaces with intriguing functionalities. Spontaneous wrinkling of film-substrate bilayer opens up a new possibility to create heterogeneous surfaces. In this work, we prepared the gradient changing wrinkle surfaces of Ag films and Mo films with identical thickness gradient on polydimethylsiloxane substrates by magnetron sputtering. It is revealed that Ag films show only labyrinth patterns, regardless of thickness gradient, while Mo films exhibit the pattern transition from labyrinth to herringbone along the thickness gradient direction. This can be attributed to the combined effect of the compressive stress, the thickness gradient and material property. The wavelength of Ag films and Mo films increases with increasing distance. Ag films show isotropic friction, which increases with increasing distance due to the increased surface roughness. In contrast, the friction properties of Mo films exhibit two-regime behavior, namely, isotropic friction and anisotropic friction, depending on the wrinkling patterns. The isotropic/anisotropic friction is closely related to the orientation-dependent topography. These results provide better understanding of the combined effect of thickness gradient and film constituent on the gradient changing wrinkle patterns and pave a new way for designing gradient surfaces with controlled friction behavior by harnessing wrinkling instability.