Abstract

Gecko adhesive performance increases as relative humidity increases. Two primary mechanisms can explain this result: capillary adhesion and increased contact area via material softening. Both hypotheses consider variable relative humidity, but neither fully explains the interactive effects of temperature and relative humidity on live gecko adhesion. In this study, we used live tokay geckos (Gekko gecko) and a gecko-inspired synthetic adhesive to investigate the roles of capillary adhesion and material softening on gecko adhesive performance. The results of our study suggest that both capillary adhesion and material softening contribute to overall gecko adhesion, but the relative contribution of each depends on the environmental context. Specifically, capillary adhesion dominates on hydrophilic substrates, and material softening dominates on hydrophobic substrates. At low temperature (12 °C), both capillary adhesion and material softening likely produce high adhesion across a range of relative humidity values. At high temperature (32 °C), material softening plays a dominant role in adhesive performance at an intermediate relative humidity (i.e., 70% RH).

Highlights

  • Gecko adhesive performance increases as relative humidity increases

  • Shear adhesion of live geckos and gecko-inspired synthetic adhesive (GSA) differed in response to temperature, relative humidity (RH), substrate wettability, and modulus (GSAs only)

  • The results of this study suggest that when geckos interact with environmental substrates that facilitate the formation of RH-induced water layers, capillary adhesion enhances adhesion

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Summary

Introduction

Gecko adhesive performance increases as relative humidity increases. Two primary mechanisms can explain this result: capillary adhesion and increased contact area via material softening. At high humidity (> 70% RH) the setal material (primarily keratin associated proteins and l­ipids35–38) softens, supporting the hypothesis that soft setae increase the interfacial contact area and subsequently increase van der Waals ­forces[19,20]. The results of these studies are difficult to ­reconcile[14,34,39], and none consider the possibility that capillary adhesion and material softening are not mutually exclusive. A nanoscopic length scale model attempted to explain the temperature dependency of this relationship, but the coupled effects of RH and temperature on gecko adhesion are still ­undetermined[39]

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