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Effect of Subsurface Microstructures on Adhesion of Highly Confined Elastic Films
Polymer adhesive films sandwiched between two rigid solids are a common bonding strategy. The mechanics and consequently the adhesion of such geometrically confined films depend mainly on their thickness, Young's modulus, and the Poisson's ratio of the material. In this work, we explore the effect of a micropatterned subsurface embedded into the adhesive layer. We compare experiments with three-dimensional numerical simulations to evaluate the impact of the microstructure on the contact stiffness and effective modulus. The results are used to extend a previously proposed size scaling argument on adhesion from incompressible to slightly compressible films to account for the silicone used in our study with a Poisson's ratio of 0.495. In addition, interfacial stress distributions between the elastic film and the glass disc are obtained from plane strain simulations to evaluate characteristic adhesion failures such as edge cracks and cavitation. Overall, the micropatterned subsurface has a large impact on the contact stiffness, the interfacial stress distribution, and the detachment behavior; however, the adhesion performance is only slightly improved in comparison to a non-patterned subsurface.
In Situ Observation Reveals Local Detachment Mechanisms and Suction Effects in Micropatterned Adhesives
Fibrillar adhesion pads of insects and geckoes have inspired the design of high-performance adhesives enabling a new generation of handling devices. Despite much progress over the last decade, the current understanding of these adhesives is limited to single contact pillars and the behavior of whole arrays is largely unexplored. In the study reported here, a novel approach is taken to gain insight into the detachment mechanisms of whole micropatterned arrays. Individual contacts are imaged by frustrated total internal reflection, allowing in situ observation of contact formation and separation during adhesion tests. The detachment of arrays is found to be governed by the distributed adhesion strength of individual pillars, but no collaborative effect mediated by elastic interactions can be detected. At the maximal force, about 30% of the mushroom structures are already detached. The adhesive forces decrease with reduced air pressure by 20% for the smooth and by 6% for the rough specimen. These contributions are attributed to a suction effect, whose strength depends critically on interfacial defects controlling the sealing quality of the contact. This dominates the detachment process and the resulting adhesion strength. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
A Design Strategy for Mushroom-Shaped Microfibrils With Optimized Dry Adhesion: Experiments and Finite Element Analyses
Enhanced dry adhesion of micropatterned polymeric surfaces has been frequently demonstrated. Among the design parameters, the cap geometry plays an important role to improve their performance. In this study, we combined experiments on single polyurethane mushroom-shaped fibrils (with a stalk diameter of 80 µm and height of 125 µm) against flat glass, with numerical simulations implementing a cohesive zone. We found that the geometry of the mushroom cap strongly affects the interfacial crack behavior and the pull-off stress. The experimental and numerical results suggest that optimal adhesion was accompanied by the appearance of both edge and interior interfacial cracks during separation. Finite elemental analyses revealed the evolution of the interfacial stress distributions as a function of the cap thickness and confirmed the distinct detachment mechanisms. Furthermore, the effect of the stalk diameter and the Young's modulus on the adhesive force was established, resulting in an optimal design for mushroom-shaped fibrils.
Bioinspired Underwater Adhesion to Rough Substrates by Cavity Collapse of Cupped Microstructures
Underwater or wet adhesion is highly desirable for numerous applications but is counteracted by the liquids in the contact which weaken intermolecular attraction. The problem is exacerbated in conjunction with surface roughness when liquids partially remain in grooves or dimples of the substrate. In the present study, a cupped microstructure with a cavity inspired by suction organs of aquatic animals is proposed. The microstructures (cup radius of 100 µm) are made from polyurethane using two-photon lithography followed by replica molding. Adhesion to rough substrates is emulated experimentally by a micropatterned model substrate with varying channel widths. Pull-off stresses are found to be about 200 kPa, i.e., twice atmospheric pressure. Evaluation of force–displacement curves together with in situ observations reveal the adhesion mechanism, which involves adaptation to surface roughness and an elastic force induced by the collapse of the cavity that holds sealed contact with the substrate during retraction. This new microarchitecture may pave the way for next generation microstructures applicable to real, rough surfaces under wet conditions.
Switchable Underwater Adhesion by Deformable Cupped Microstructures
Switchable underwater adhesion can be useful for numerous applications, but is extremely challenging due to the presence of water at the contact interface. Here, deformable cupped microstructures (diameter typically 100 µm, rim thickness 5 µm) are reported that can switch between high (≈1 MPa) and low (<0.2 MPa) adhesion strength by adjusting the retraction velocity from 100 to 0.1 µm s–1. The velocity at which the switch occurs is determined by specific design parameters of the cupped microstructure, such as the cup width and angle. The results are compared with theoretical estimates of water penetration into the contact zone and expansion of the cup during retraction. This work paves the way for controlling wet adhesion on demand and may inspire further applications in smart adhesives.