Electrostatic attraction caused by triboelectrification in climbing geckos
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ISSN 2223-7690 CN 10-1237/TH
RESEARCH ARTICLE
Electrostatic attraction caused by triboelectrification in climbing geckos Yi SONG, Zhouyi WANG, Yang LI, Zhendong DAI* College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China Received: 27 November 2019 / Revised: 10 February 2020 / Accepted: 20 March 2020
© The author(s) 2020. Abstract: 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 ~1/70th 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/mm 2. 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. Keywords: biotribocharge; electrostatic force; geckos; adhesion
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Introduction
Geckos are recognized as one of the most excellent climbers that can maneuver on various terrains in all orientations [1–5]. Their stride frequencies can be as large as a dozen Hertz, and their speeds could be as fast as ~1 m/s. Previous studies have attributed the versatility of geckos in locomotion to their abilities to alter body dynamics and rapidly obtain proper adhesion through their compliant, hierarchical, and hairy footpads [6–8]. Therefore, understanding the physics of geckos’ feet and the mechanism of
adhesion generation is of great significance to understand the biological attachment mechanism and inspire artificial adhesives. Attempts aiming at determining the adhesion mechanisms of geckos can be traced back to the 1800s. In the last two centuries, many hypotheses, including adhesive secretion, suction, friction, and micro-interlocking, have bee
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