Design of bio-inspired directional tapered adhesives and hierarchies [electronic resource]
Keywords: Mechanical Engineering

Research into how the gecko lizard is able to climb a wide variety of surfaces has re- vealed an adhesive system that takes a fundamentally different approach than is found in conventional pressure-sensitive adhesives such as sticky tape. The gecko's adhesive system is composed of setal stalks, each thinner than a human hair and terminating in spatulae only 250 nm across. The entire hierarchical system is composed of beta- keratin, a tough, hydrophobic material, somewhat harder than the alpha-keratin of human fingernails. The geometry of the setae and spatulae allow them to conform to surfaces in a manner similar to very soft materials, but without the tendency of tacky materials to become fouled with dirt. Using the gecko adhesive system as inspiration, Biomimetics and Dexterous Ma- nipulation Laboratory developed an adhesive that is suitable for robotic climbing ap- plications. The smallest features of this adhesive are arrays of sharp wedges molded from silicone rubber. A tapered feature was pursued because it is capable of repro- ducing the "frictional adhesion" property of the gecko's adhesive system. Frictional- adhesion defines a behavior for which increasing the shear stress imposed at a contact increases the available adhesive stress perpendicular to the surface. A consequence of frictional adhesion is that one can control the amount of adhesion by controlling the applied shear load. In the present case, the behavior arises from the fact that sharp wedge-shaped features initially present very little area as they are brought into contact with a surface. However, they bend over when the array is loaded in shear, so that the contact area and the adhesion grow in proportion. This thesis seeks to understand how the details of the tapered wedge geometry, including the wedge profile and angle of inclination, influence the frictional adhesive behavior. The analysis includes a combination of numerical finite element modeling and empirical pull-off tests. The constraints on material stiffness, wedge geometry and spacing are also studied, as affected by possible failure modes such as self-sticking of adjacent wedges (leading to "clumping"). The desire to test wedges at various angles of inclination lead to the development of a new micro-machining process for creating molds for the wedge arrays. This process affords much greater freedom to control the wedge size and geometry than a previous lithographic process. However, a byproduct of the machining process is that the wedges have a non-negligible surface roughness on their contacting faces, which compromises their performance. Consequently, a new process was developed to improve the surface finish by "inking" the molded wedges, depositing a thin film of liquid silicone rubber onto their faces and providing a smoother surface. The resulting microwedges achieve more than double the maximum adhesion and several times the adhesion at low levels of shear than previous microwedges from molds created using the lithographic process. Although the microwedges stick well to smooth, flat surfaces such as glass, they cannot conform to surfaces with undulations higher than a couple of micrometers. In addition, the array of microwedges must be precisely aligned with surfaces so that all wedges are uniformly loaded. To mitigate these limitations, some approximation to the gecko's compliant hierarchy of lamellae, setae and spatulae is needed. The solution presented in this thesis is a two-layer hierarchical system in which the arrays of wedges are supported by a larger array of angled pillars. In between the pillars and wedges is a film of solid silicone rubber, which bridges the gaps between pillars and helps to create a relatively uniform loading of the wedges. A combination of numerical analysis and empirical pull-off tests is used to understand the relationships among pillar dimensions, pillar spacing and film thickness that govern the performance of this structure. At one extreme, the loading can become sufficiently non-uniform that some wedges lose contact with the surface, resulting in a loss of adhesion. At the other extreme, the structure is too stiff to accommodate surface undulations and misalignment. The thesis concludes with a summary of the results on wedges and hierarchical adhesive structures, and discusses the implications for future work.