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Issue 44, 2015
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Rate-dependence of ‘wet’ biological adhesives and the function of the pad secretion in insects

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Abstract

Many insects use soft adhesive footpads for climbing. The surface contact of these organs is mediated by small volumes of a liquid secretion, which forms thin films in the contact zone. Here, we investigate the role of viscous dissipation by this secretion and the ‘bulk’ pad cuticle by quantifying the rate-dependence of the adhesive force of individual pads. Adhesion increased with retraction speed, but this effect was independent of the amount of pad secretion present in the contact zone, suggesting that the secretion's viscosity did not play a significant role. Instead, the rate-dependence can be explained by relating the strain energy release rate to the speed of crack propagation, using an established empirical power law. The ‘wet’ pads' behaviour was akin to that of ‘dry’ elastomers, with an equilibrium energy release rate close to that of dry van-der-Waals contacts. We suggest that the secretion mainly serves as a ‘release layer’, minimising viscous dissipation and thereby reducing the time- and ‘loading-history’-dependence of the adhesive pads. In contrast to many commercial adhesives which derive much of their strength from viscous dissipation, we show that the major modulator of adhesive strength in ‘wet’ biological adhesive pads is friction, exhibiting a much larger effect than retraction speed. A comparison between ‘wet’ and ‘dry’ biological adhesives, using both results from this study and the literature, revealed a striking lack of differences in attachment performance under varying experimental conditions. Together, these results suggest that ‘wet’ and ‘dry’ biological adhesives may be more similar than previously thought.

Graphical abstract: Rate-dependence of ‘wet’ biological adhesives and the function of the pad secretion in insects

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Supplementary files

Article information


Submitted
17 Jun 2015
Accepted
10 Sep 2015
First published
10 Sep 2015

This article is Open Access

Soft Matter, 2015,11, 8661-8673
Article type
Paper
Author version available

Rate-dependence of ‘wet’ biological adhesives and the function of the pad secretion in insects

D. Labonte and W. Federle, Soft Matter, 2015, 11, 8661 DOI: 10.1039/C5SM01496D

This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. You can use material from this article in other publications without requesting further permissions from the RSC, provided that the correct acknowledgement is given.

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