The Role of Surface Chemistry in Adhesion and Wetting of Gecko Toe Pads

Abstract

An array of micron-sized setal hairs offers geckos a unique ability to walk on vertical surfaces using van der Waals interactions. Although many studies have focused on the role of surface morphology of the hairs, very little is known about the role of surface chemistry on wetting and adhesion. We expect that both surface chemistry and morphology are important, not only to achieve optimum dry adhesion but also for increased efficiency in self-cleaning of water and adhesion under wet conditions. Here, we used a plasma-based vapor deposition process to coat the hairy patterns on gecko toe pad sheds with polar and non-polar coatings without significantly perturbing the setal morphology. By a comparison of wetting across treatments, we show that the intrinsic surface of gecko setae has a water contact angle between 70–90°. As expected, under wet conditions, adhesion on a hydrophilic surface (glass) was lower than that on a hydrophobic surface (alkyl-silane monolayer on glass). Surprisingly under wet and dry conditions the adhesion was comparable on the hydrophobic surface, independent of the surface chemistry of the setal hairs. This work highlights the need to utilize morphology and surface chemistry in developing successful synthetic adhesives with desirable adhesion and self-cleaning properties.

Adhesive interactions of geckos with wet and dry fluoropolymer substrates

“…We included the air -water surface tension term to account for air trapped between the setal structures in our model unit cell. This is slightly different than the calculations from Stark et al [8] and is explained further by Badge et al [9]. Taking into account these assumptions, the ratio of the work of adhesion in water and the work of adhesion in air (W wet : W dry ) of a structured surface is…”

“…This focus has been quite successful. In recent studies, however, the role of substrate and surface chemistry on gecko adhesion in the presence of water has been investigated, and the results are not yet clear [8,9,14,46,47]. We argue that turning our attention to these aspects is likely to improve design and performance Table 1.…”

“…The mechanism behind biological adhesive systems can be complex however. Specifically, when testing geckos underwater, shear adhesion no longer relies solely on van der Waals interactions, and instead also depends on the surface energy of the substrate and its interaction with water [8]; but interestingly, not the surface chemistry of the setae [9]. Other recent studies have suggested that van der Waals forces are not the primary source of adhesion and rather are also coupled with capillary adhesion, electrostatic interactions, surface charge or even nanobubbles underwater [10][11][12][13][14][15].…”

Recent biomedical applications of bio-sourced materials

“…As a common reptile in nature, geckos can walk on smooth walls and can even adhere to the ceiling and crawl quickly [45,47]. These characteristics are caused by the combination of the size effect of the micro-/nanosetae and sophisticated motion control geckos, using the adductor and valgus of foot hallux that ultimately enable it to stand and walk on walls and ceilings [ Fig.…”

Sticking to the story: outstanding challenges in gecko-inspired adhesives

“…However, only a handful of studies have examined gecko adhesion on wet substrates, and the results are not simple to understand. Indeed, in addition to the way humidity can affect the mechanical properties of setae, the effects of water at the contact interface also vary depending upon the surface chemistry of the substrate, substrate roughness and whether the setae are in their default un-wetted, superhydrophobic state (see Glossary) (Stark et al, , 2013(Stark et al, , 2014a(Stark et al, ,b, 2015aBadge et al, 2014). The default wetting state for gecko setae is that they repel water, but that state is not stable.…”