What is the Difference between Contact and Sliding? Tangential Forces are Important for Triboelectricity

Despite increasing research interest in triboelectricity, the electric charge transfer that occurs when two materials contact each other, the fundamental understanding of the subject is still limited, especially when at least one of the materials involved is insulating. When surface asperities of the rubbing materials contact each other, they deform, resulting in electromechanical effects that are large enough to cause charge transfer. Especially important is flexoelectricity, the coupling between electric polarization and the strain gradient, which can be very large due to the small size of the asperities [1].<br/><br/>Previously, we have modeled the electromechanics of simple single-asperity metal-semiconductor contacts [2]. Here, we extend this analysis to consider tangential forces as well as normal forces. Macroscopically, this roughly corresponds to sliding instead of simple contact and release.<br/><br/>Intuitively, one might suspect that sliding a balloon on hair causes more charge transfer than contacting and separating. Indeed, the results of our work suggest that in some cases, sliding forces can be more impactful than normal forces. The relative importance of normal forces and tangential forces to the electromechanics of contacting asperities is dependent on material parameters. This fundamental insight into the importance of the geometry of the contact force may be useful both in cases where charge transfer should be prevented to reduce dangerous sparks or where it is desired to power triboelectric nanogenerators.<br/><br/>[1] Mizzi, C.A.; Marks, L.D. “When Flexoelectricity Drives Triboelectricity”. <i>Nano Lett.</i> 2022, <b>22</b>, 3939-3945.<br/>[2] Olson, K.P.; Mizzi, C.A.; Marks, L.D. “Band Bending and Ratcheting Explain Triboelectricity in a Flexoelectric Contact Diode”. <i>Nano Lett. </i>2022, <b>22</b>, 3914-3921.