Origin of the superior adhesive performance of mushroom-shaped microstructured surfaces

Abstract

The superlative adhesive properties of some biological attachment systems, such as those of geckos, spiders, and insects, have inspired researchers from different fields (e.g. biology, physics and engineering) to conceive and design man-made microstructured surfaces that might mimic their performance. Among the several proposed designs, very recently mushroom-shaped adhesive microstructures have drawn the interest of scientists and engineers, because experiments have proved their superiority compared to other micro- and nano-structures. In this article, we explain theoretically the physical mechanism behind the enhanced adhesion of such microstructures, and provide for the first time a useful tool to predict adhesive performance depending on the geometry, mechanical properties of the material, and energy of adhesion. Our theoretical predictions are strongly supported by the available experimental data. The present study can streamline the optimisation of adhesive microstructures for industrial applications

Rectangle-capped and tilted micropillar array for enhanced anisotropic anti-shearing in biomimetic adhesion

“…Significantly, increase in normal adhesion for capped micropillars is owing to the laterally bulged tips, which increase the contact area while maintaining flexibility of the structures while also increasing tolerance for tiny defects owing to the overhanging portion of the caps. The contact tip of capped micropillars can reduce or even eliminate edge stress concentration of the stalk [22], which is useful to prohibit crack initiation of the tip from the silicon surface and crack propagation, leading to an enhanced adhesive strength. Although, vertical and slanted micropillars with identical rectangle-capped tips have similar saturated adhesion strength at a large preload, slanted micropillars demonstrate better adhesion at low preloads (less than 0.6 N cm 22 ), compared with vertical micropillars.…”

“…The contact tip of capped micropillars can reduce or even eliminate edge stress concentration of the stalk [22], which is useful to prohibit crack initiation of the tip from the silicon surface and crack propagation, leading to an enhanced adhesive strength. Although, vertical and slanted micropillars with identical rectangle-capped tips have similar saturated adhesion strength at a large preload, slanted micropillars demonstrate better adhesion at low preloads (less than 0.6 N cm 22 ), compared with vertical micropillars. This observation can be explained using a mechanical model of the micropillar cantilever presented by Autumn et al [30].…”

“…These nanohair surfaces showed strong anisotropic interfacial strength [20]. The bulged flat tips of the micropillars presented in these attempts were approximately round in their planar shape, and were mostly intended to enhance the interfacial strength itself by an increased actual contact area of the microstructured adhesive on the target surface [22]. As a matter of fact, the shape of the tip endings alone can also affect the anisotropy of the artificial adhesive, as has been discussed in reports presented in the literature by other research groups [25,26].…”

“…However, these artificial adhesives with inclined micropillars demonstrated a relatively lower magnitude of interfacial friction and adhesion. This was because the micropillars did not have a laterally bulged, flat top tip, which has been regarded as an effective mechanism for enhancing adhesion and friction [4,[21][22][23][24]. To overcome this, Murphy et al proposed a micrometre-scale structure using an array of mushroom-shaped fibres, with titled stalks and laterally extended tip ends.…”

Tangential Displacement Influence on the Critical Normal Force of Adhesive Contact Breakage in Biological Systems

“…The fibers' elasticity together with their endings' good adhesion to surfaces allows the gecko to easily attach to the majority of natural surfaces. Based on the gecko's feet structure as a natural prototype, a scientific group around S. Gorb [2] has created an artificial material with a similar structure, which "sticks" to almost any surfaces.…”