The function of the pad secretion in insects

节肢动物脚上的粘性垫在几个方面优于人造粘合剂：它们可以在短时间内控制，在数千次使用周期中生存而不降低性能，在粗糙和潮湿的表面上工作，并在污染后恢复功能。因此，了解这些垫的结构-功能关系有望揭示具有新特性的技术粘合剂设计的有效策略。节肢动物粘合剂的一个特点是它们是“湿的”--柔软的脚垫和表面之间的接触是由纳米级的油性分泌物薄膜介导的。尽管这种液体在近200年前就被发现了，但其功能仍不清楚，最近的实验表明，脚垫的粘合性能可能受固体脚垫本身的性质所支配，重新引发了关于液体功能的争论-为什么昆虫会分泌这种液体？它是否作为一个保护层，防止磨损，或它是否防止脱水垫本身，这将导致增加垫刚度，从而降低适应性粗糙表面？其他人认为，流体对于控制附着力至关重要：它可以用作“释放层”，确保可以轻松地进行快速自主分离，但是当在伴随强制分离的快速滑动事件期间流体从接触区耗尽时，会发生强附着。在这个项目中，我们解决这些假设，通过量化的脚垫和粘合剂分泌物在单独的实验中的相关物理特性：的粘弹性的附着垫将使用压痕和国家的最先进的粘弹性建模量化，而粘性和垫分泌物的表面张力将测量与去湿实验和接触角测量的组合。然后，所获得的信息将用于构建和告知数学模型，这将使我们能够预测衬垫的粘合性能。通过比较这些理论预测的生物粘附垫的粘附在垫分泌的情况下和存在下的速率依赖性的实验数据，我们将能够分离的流体和垫的粘弹性的贡献所观察到的粘合性能，从而增加我们的了解垫分泌的功能意义。因此，我们的研究将不仅有助于解决一个古老的难题，为软固体和薄液体膜之间的机械耦合提供新的基本见解，而且重要的是产生与新型人造粘合剂的设计相关的结果，这些粘合剂成功地复制了动态生物粘合剂的一些最杰出的性能特征。

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.