Abstract
Biological adhesive systems in both geckoes and climbing plants share similar hierarchical structures, such as the toe-seta-spatula structure in geckoes and the root-rootlet-hair structure in English ivy (Hedera helix). The former operates at a spectrum of length scales that are much smaller than the latter. Consequently, the spatula adhesion in geckoes exhibits a flaw-insensitive behavior, or in other words, the large-scale-bridging characteristics shield the stress singularities at the adhesive contact front. In contrast, adventitious root hairs from commonly seen household climbing plants are of several tens to hundreds of micrometers long, so that the adhesive contact appears to resemble a linear elastic crack and thus would have a very low pulling force for de-adhesion. This apparent contradiction between modeling and observations is resolved in this work by a coupled transport–adhesion mechanism, in which an adhesive layer that carries gluing nanoparticles flows towards the adhesive contact front. This provides an effective way to shield the stress singularity, resulting in a scenario that completely differs from gecko adhesion. Finite element simulations have been conducted to illustrate this proposed mechanism and then compared to available experimental observations in the literature.