The function of the pad secretion in insects

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The sticky pads on the feet of arthropods outperform man-made adhesives in several aspects: they can be controlled on short time-scales, survive thousands of usage cycles without a decline in performance, work on rough and wet surfaces, and regain functionality after contamination. Understanding the structure-function relationship of these pads hence promises to reveal effective strategies for the design of technical adhesives with novel properties. A hallmark of arthropod adhesives is that they are "wet" - the contact between the soft footpads and a surface is mediated by nano-metre thin films of an oily secretion. Despite its discovery almost 200 years ago, the function of this fluid is still unclear, and recent experiments have suggested that the adhesive performance of the pads may be dominated by the properties of the solid footpad itself, reigniting the debate about the fluid's function - why do insects secrete this fluid? Does it serve as a protective layer against wear, or does it prevent dehydration of the pad itself, which would result in an increased pad stiffness, and hence reduced adaptability to rough surfaces? Others have argued that the fluid is crucial for the control of attachment forces: it may serve as a "release layer" which ensures that fast voluntary detachment can be performed with ease, but that strong attachment occurs when the fluid is depleted from the contact zone during the fast sliding events that accompany forced detachment. In this project, we address these hypotheses by quantifying the relevant physical properties of both the footpads and the adhesive secretion in separate experiments: the viscoelastic properties of the attachment pads will be quantified using indentation and state-of-the-art viscoelastic modelling, while the viscosity and surface tension of the pad secretion will be measured with a combination of dewetting experiments and contact angle measurements. The obtained information will then be used to construct and inform mathematical models which will allow us to predict the pads' adhesive performance. By comparing these theoretical predictions to experimental data on the rate-dependence of the adhesion of the biological adhesive pads in the absence and presence of the pad secretion, we will be able to separate the contribution of both the fluid and the pad viscoelasticity to the observed adhesive performance, and thereby increase our understanding of the functional significance of the pad secretion. Our study will hence not only help to solve a centuries-old puzzle, provide new fundamental insights into the mechanical coupling between soft solids and thin liquid films, but importantly yield results relevant for the design of novel man-made adhesives which successfully replicate some of the most outstanding performance characteristics of dynamic biological adhesives.