2018, Hensel, René, Moh, Karsten, Arzt, Eduard

Reversible adhesion is the key functionality to grip, place, and release objects nondestructively. Inspired by nature, micropatterned dry adhesives are promising candidates for this purpose and have attracted the attention of research groups worldwide. Their enhanced adhesion compared to nonpatterned surfaces is frequently demonstrated. An important conclusion is that the contact mechanics involved is at least as important as the surface energy and chemistry. In this paper, the roles of the contact geometry and mechanical properties are reviewed. With a focus on applications, the effects of substrate roughness and of temperature variations, and the long-term performance of micropatterned adhesives are discussed. The paper provides a link between the current, detailed understanding of micropatterned adhesives and emerging applications.

2019, Tinnemann, Verena, Hernández, Luissé, Fischer, Sarah C.L., Arzt, Eduard, Bennewitz, Roland, Hensel, René

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

2015, Bauer, Christina T, Kroner, Elmar, Fleck, Norman A, Arzt, Eduard

Nature uses hierarchical fibrillar structures to mediate temporary adhesion to arbitrary substrates. Such structures provide high compliance such that the flat fibril tips can be better positioned with respect to asperities of a wavy rough substrate. We investigated the buckling and adhesion of hierarchically structured adhesives in contact with flat smooth, flat rough and wavy rough substrates. A macroscopic model for the structural adhesive was fabricated by molding polydimethylsiloxane into pillars of diameter in the range of 0.3–4.8 mm, with up to three different hierarchy levels. Both flat-ended and mushroom-shaped hierarchical samples buckled at preloads one quarter that of the single level structures. We explain this behavior by a change in the buckling mode; buckling leads to a loss of contact and diminishes adhesion. Our results indicate that hierarchical structures can have a strong influence on the degree of adhesion on both flat and wavy substrates. Strategies are discussed that achieve highly compliant substrates which adhere to rough substrates.