The impact of asperity shape and gradient elasticity in flexoelectric/triboelectric contacts

Flexoelectricity, the coupling of strain gradients and polarization, is necessary to explain the charge transfer that occurs when an insulator contacts another material, known as triboelectricity. When asperities at the surface of materials contact, they deform with a large strain gradient, which drives charge transfer via the flexoelectric effect [1]. Previously, we have developed an experimentally verified model for metal-semiconductor contacts which explains the force-dependence of current when an atomic force microscope tip is used to deform the semiconductor [2].

Here, the model is used to investigate some critical questions related to the contact mechanics of triboelectricity: how important the shape of the asperities is and when gradient elasticity must be considered. The results indicate that considering asperity shape, gradient elasticity, or both is essential in some cases and practically insignificant in others. Additionally, we show that how the key electromechanical quantities scale with the contact force and geometry of the asperity is dependent on the asperity shape. This provides insight into the fundamentals of triboelectricity and presents a clear challenges for a new set of experiments.

[1] Mizzi, C.A.; Marks, L.D. “When Flexoelectricity Drives Triboelectricity”. Nano Lett. 2022, 22, 3939-3945.

[2] Olson, K.P.; Mizzi, C.A.; Marks, L.D. “Band Bending and Ratcheting Explain Triboelectricity in a Flexoelectric Contact Diode”. Nano Lett. 2022, 22, 3914-3921.