Contact area shrinkage and increase in wavy frictional sliding contacts

The effect of material and geometric nonlinearity is often underestimated in contact mechanics. However, recent experiments reveal that classical linear models might fail to accurately predict key contact features, such as the contact area, in scenarios involving frictional sliding. In this study, we employ accurate yet simple plane-strain finite element simulations to investigate frictional sliding contact under finite elasticity. We consider both rigid and deformable sinusoidal indenters pressed against a flat substrate, exploring both periodic and aperiodic boundary conditions. Our results show that the transition of the contact area from the static conditions to the gross sliding is qualitatively governed by the pressure value. Indeed, at low pressure contact shrinkage is observed, in agreement with most experimental observations led under qualitatively similar pressure levels. Importantly, we also found a pressure threshold above which the sliding contact area can exceed the static one, especially for deformable sinusoids with high aspect ratio. To validate our numerical results, we perform ad hoc experiments with micro-fabricated soft sinusoids in either static or sliding contact against a microscope slide, which confirm the trend. Moreover, we also investigate the role of periodic boundary conditions, showing that this is not a key factor and aperiodic contacts behave almost the same. These novel findings provide deeper insights into rubber nonlinear contact mechanics at the sinusoid scale, which constitutes the building block of rough contact mechanics, showing that contact area increase is also possible without adhesion, with direct implications for real tribological systems such as tire-road and seal interactions, soft robotics locomotion, and biomechanics.