Oscillating friction on shape-tunable wrinkles.

Friction on soft materials is strongly correlated with the associated deformation, which may be controlled by the surface topography. We investigate the wearless sliding friction between a rigid hemispherical indenter and a deformable textured surface, which is shape-tunable wrinkles. The size of the indenter is comparable to the wavelength of the wrinkles. We evaluate the effects on the friction of the aspect ratio of the wrinkles, the applied normal load, and the alignment direction of the wrinkles relative to the sliding direction. The frictional oscillations are observed during sliding in the direction perpendicular to the alignment using optical images and friction profiles. The correlation of friction force oscillation with deformation of the wrinkles is elucidated using Hertz contact theory. Within a cycle of frictional oscillation, the friction force increases as the front part of the indenter elastically plows the crests. When the normal load is high and/or the aspect ratio of the wrinkles is low, the indenter continues to squash the wrinkles and remains in contact with them during sliding. Consequently, the amplitude of friction force oscillation relative to the averaged friction force decreases.