In nature, several animal groups, like insects, arachnids, tree frogs, and lizards use reversible adhesions for their locomotion. Comprehensive studies have suggested two highly specialized structures, which are smooth pads and fibrillar pads, rather than the surface chemistry of the toe pads contribute mainly to the strong adhesion of these animals. While geckos and spiders bear fibrillary adhesives, tree frogs and locusts have smooth pads. From the geometrical point of view, these two kinds of adhesive pads can be considered as pillar arrays, but with different aspect ratios and hierarchies. A larger aspect ratio of pillars could increase the possibility to form contacts on various substrates. The direct contacts between pillars and solid surface generates van der Waals forces for the adhesion. Moreover, the space among the pillars allows the liquid to be drained out from the contact area, forming the direct contacts, though this kind of adhesion is normally considered as wet adhesion as liquid is involved at the contact interface.
Inspired by the amazing adhesion abilities of these animals, hierarchical micro- & nano-pillar arrays composed of materials with elastic modulus ranging from few MPa to ~1 TPa have been developed. It has found that the tip geometry of the pillars is another crucial parameter to control the adhesion performance. The presence of overhang structure on top of pillars could change the stress distribution at the contact interface and enlarge the contact areas. With a proper geometry, the overhang structure could shift the stress maximum to the center region of contact are and greatly reduce the stress at the contact perimeter. This kind of stress distribution could inhibit the initiation of cracks during detachments and therefore contributes to the adhesion enhancement. Recently, it has also been found that the rigid nanopillars inside micropillar array could also alter the distribution of interfacial stress. The stress maximum was distributed on top of the embedded nanopillars, some distance away from the contact perimeter. Moreover, the stress minimum was located at the contact perimeter. It therefore results in a strong adhesion and friction.
By optimizing the sub-structure in micropillar arrays, the stress at contact interface could be regulated, contributing to the establishment of reliable and reversible adhesions and frictions, which may find wide applications in daily life, industry, and so on.