Abstract
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.
Highlights
Animals such as various species of insects, spiders and lizards, can adhere to different kinds of substrates [1,2,3,4,5,6]
Our results indicate that hierarchical structures can have a strong influence on the degree of adhesion on both flat and wavy substrates
Several research groups have developed artificial gecko-inspired adhesion substrates [15,16,17,18,19,20,21,22,23,24] or even hierarchical structures [25,26,27,28,29,30,31,32,33,34], but only few studies exist on bioinspired adhesion structures on rough substrates [35,36,37,38]; some papers address adhesion of an artificial hierarchical system to rough substrates [39,40,41,42,43,44,45] and experiments with living geckos on engineered rough substrates has been made [46]
Summary
Animals such as various species of insects, spiders and lizards, can adhere to different kinds of substrates [1,2,3,4,5,6] They have developed hairy attachment systems which enable them to stick to a wide range of substrate roughness. The gecko, for this purpose, possesses a hairy dry adhesion system with at least three levels of hierarchy [7,8,9,10]: the toe pad substrate consists of lamellae covered with setae, which branch into even finer spatulae. Several theoretical studies suggest that the introduction of structural hierarchy increases adhesion to rough substrates [48,49,50], but experimental evidence is lacking
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