Browsing by Author "Yao, Haimin"

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Mechanics of robust and releasable adhesion in biology
(2006) Yao, Haimin; Gao, Huajian (Prof. Dr.)
Nature has found, through billions years of natural evolution, many ingenious ways to produce materials with superior mechanical properties. It would be a convenient and practical way for us to explore the existing biological systems for the ideas of designing novel materials. In this thesis, our attention will be focused on dry adhesion, a specific phenomenon observed frequently in many animal species like gecko, fly and insects. Our goal is to elucidate the adhesion mechanism behind these professional climbers. The prospective results may provide a useful guideline for the fabrication of the novel materials and devices in engineering. The whole thesis can be broadly divided into three parts. In part one, which consists of Chapter 1-3, we begin by introducing the motivation and overview of this work. Then, in Chapter 2 the theory of contact mechanics is reviewed briefly. It is followed by Chapter 3 in which the useful research methods are introduced with emphasis on the mathematical preliminaries and computational methods. For animals like gecko, the robustness and releasability of the attachment systems are two essential features ensuring their locomotion on vertical walls or ceilings. The second part of this thesis (Chapter 4-6) is entirely devoted to the investigation on these two seemingly contradictive characteristics. While the adhesion robustness is treated in Chapters 4 and 5, adhesion releasability is the subject of Chapter 6. Given contact area, adhesion strength is commonly measured by the magnitude of the pull-off force, i.e., the force required to pull two bonded objects apart. The higher the pull-off force, the stronger is the adhesive joint. How to increase the adhesion strength as much as possible is what we are interested in. In Chapter 4, we start with study on the adhesion between two single contact asperities. It is found that the adhesion strength is strongly dependent on the geometric shape and size of the contact surfaces and limited by the theoretical strength. There exists a specific shape termed optimal shape, by which theoretical adhesion strength can be achieved. A general methodology for determining the optimal shape is developed, by which analytical expressions of the optimal shapes for several example cases are obtained. However, shape optimization design for optimum adhesion is found to be unreliable especially at the macroscopic scale because the pull-off force then is quite sensitive to the small variations in the contact shape. A robust design of shape-insensitive optimal adhesion becomes possible only when the characteristic size of the contact area is reduced to a length scale on the order of 100 nm. In general, optimal adhesion could be achieved by a combination of size reduction and shape optimization. The smaller the size, the less important the shape. It is basically for this reason that the fibrillar nanostructures in biology possess high adhesion strength. The results obtained in Chapter 4 imply that materials have intrinsic ability to tolerate quite small contact flaws. In reality, however, contact surfaces tend to be rough in a variety of length scales, leading to multi-scale contact flaws. Optimizing adhesion at the level of single asperities or fibrils does not automatically address the problem of robust adhesion on rough surfaces at macroscopic scales. To solve this problem, we study, in Chapter 5, the adhesion strength between rough surfaces. Instead of directly modeling adhesive contact on random or fractal rough surfaces, we follow a different approach by considering the behavior of an interfacial crack representing random contact flaws due to surface roughness or contaminants. By investigating the conditions under which the representative crack does not grow, we effectively treat, in a statistically average sense, the problem of how to prevent randomly occurring poor contact regions from triggering crack-like adhesive failure. So that a state of flaw-tolerance is achieved in which preexisting cracks do not propagate even as the material is stretched to failure near its theoretical strength. In Chapter 5, various strategies for achieving flaw tolerant adhesion are discussed. It is found that in traditional homogeneous materials, flaw tolerance can only be achieved on condition that the structure dimension is reduced to below a critical length scale. To achieve the generalized flaw tolerance in which crack-like flaws of all sizes can be tolerated, we have to appeal to the graded material or hierarchical design. Both theoretical modeling and numerical simulation show that a graded material in conjunction with hierarchical energy dissipation mechanism can be designed to suppress the growth of interfacial cracks of all sizes so as to achieve the flaw tolerance from the smallest dimension up to macroscopic length scales. Such design philosophy also agrees well with the common structural features observed from a variety of biological attachment systems. For most animals, however, only having robust adhesion ability can not sufficiently ensure them to move on the vertical and even reverse surfaces. The releasability of the attachment devices is just as important as the robustness. Our discussion of Chapter 6 is dedicated to the problem of how to release the robust adhesion with ease. Inspired by the common structural features of the biological attachment systems, we study the effect of the material anisotropy on the orientation dependence of adhesion strength. It is found that materials with strong anisotropy allow the adhesion strength to vary strongly with the direction of pulling. The resulting orientation-dependent pull-off force enables robust attachment in the stiff direction of the material to be released just by pulling in the soft direction, achieving an orientation-controlled switch between attachment and detachment. Biological adhesion devices are sophisticated systems which provide a rich source of ideas for development of industrial applications. The concept developed in this thesis should be of general value in understanding the biological attachment devices and the design of synthetic adhesive systems in engineering. In the last part of this thesis, Chapter 7, the most important results obtained in this thesis are summarized and the whole thesis is concluded by providing an outlook to the future work.