Electrostatic attraction caused by triboelectrification in climbing geckos

Adhesion achieved through feet setae is fundamental for gecko agilely maneuvering. Although diverse hypotheses have been proposed, none of them thoroughly explains the setae function, implying a kind of hybrid-mechanism-based adhesion in geckos. In addition to van der Waals interactions and capillary force, the electrostatic attraction that emerges from triboelectrification was suggested as a component of setae adhesion. Nevertheless, the contribution by electrostatic attraction to the total setae attachment is still controversial. In this study, we analyzed the occurrence of electrostatic attraction at gecko setae through experiments and model analyses. By touching the substrates with only ~1/70^th of the foot area, freely wall-climbing geckos developed tribocharge at their feet setae with a density of ~277 pC/mm², generating electrostatic attractions with a strength of ~4.4 mN/mm². From this perspective, the adhesion driven by triboelectrification could account for about 1% of total adhesion. Model analyses at spatula level indicated a similar result showing that the electrostatic force might account for ~3% of the adhesion that facilitates wall-climbing in geckos. The low contribution of the electrostatic force partly explains why geckos always face difficulty in maneuvering onto those substrates (e.g., teflon) where they could easily develop tribocharge but difficultly generate van der Waals force. However, long-range electrostatic forces may play other roles in a distance range where the van der Waals interaction cannot function. These findings not only add to our understanding of the mechanism of gecko adhesion, but also will help us advance gecko-inspired fibular adhesives.