Non-monotonic evolution of contact area in soft contacts during incipient torsional loading

TL;DR

This study reveals that contact area in soft elastomer contacts under torsional loading evolves non-monotonically, initially increasing then decreasing, challenging existing models and highlighting the complex behavior under isotropic shear conditions.

Contribution

It demonstrates the non-monotonic evolution of contact area under torsional loading and explores conditions affecting this behavior, providing new insights into soft contact mechanics.

Findings

Contact area initially increases then decreases during torsional loading.

Maximum contact area ratio is weakly dependent on load, velocity, and dwell time.

Non-monotonic area evolution also occurs under unidirectional shear with large normal force.

Abstract

Many properties of soft contact interfaces are controlled by the contact area (e.g. friction, contact stiffness and surface charge generation). The contact area increases with the contact age at rest. In contrast, it usually reduces under unidirectional shear loading. Although the physical origin of such a reduction is still debated, it always happens in an anisotropic way because the reduction mainly occurs along the shearing direction. Whether such anisotropy is a necessary condition for shear-induced area reduction remains an open question. Here, we investigate the contact area evolution of elastomer-based sphere-plane contacts under an isotropic shear loading, i.e. torsional loading. We find that, when macroscopic sliding is reached, the contact area has undergone a net area reduction. However, the area evolves non-monotonically as the twisting angle increases, with an initial rise up to a maximum before dropping to the value during macroscopic sliding. The ratio of maximum to initial contact area is found weakly dependent on the normal load, angular velocity and dwell time (time interval between the instants when the normal load and twist motion are first applied) within the investigated ranges. We show that non-monotonic area evolution can also be found under unidirectional shear loading conditions under large normal force. These observations challenge the current descriptions of shear-induced contact area evolution and are expected to serve as a benchmark for future modelling attempts in the field.