Surface wettability plays a significant role in gecko adhesion underwater

Abstract

Although we now have thousands of studies focused on the nano-, micro-, and whole-animal mechanics of gecko adhesion on clean, dry substrates, we know relatively little about the effects of water on gecko adhesion. For many gecko species, however, rainfall frequently wets the natural surfaces they navigate. In an effort to begin closing this gap, we tested the adhesion of geckos on submerged substrates that vary in their wettability. When tested on a wet hydrophilic surface, geckos produced a significantly lower shear adhesive force (5.4 ± 1.33 N) compared with a dry hydrophilic surface (17.1 ± 3.93 N). In tests on an intermediate wetting surface and a hydrophobic surface, we found no difference in shear adhesion between dry and wet contact. Finally, in tests on polytetrafluoroethylene (PTFE), we found that geckos clung significantly better to wet PTFE (8.0 ± 1.09 N) than dry PTFE (1.6 ± 0.66 N). To help explain our results, we developed models based on thermodynamic theory of adhesion for contacting surfaces in different media and found that we can predict the ratio of shear adhesion in water to that in air. Our findings provide insight into how geckos may function in wet environments and also have significant implications for the development of a synthetic gecko mimic that retains adhesion in water.contact angle | superhydrophobicity | van der Waals | friction | bioinspired adhesive O ver the past decade, researchers have made extraordinary progress in understanding how the gecko adhesive system works (1-8). Indeed, many laboratories have tested hundreds of synthetic mimics for potential use in robotics, medicine, space, and everyday life (9-21). Although the range and performance of synthetic "gecko-tapes" are impressive, important gaps remain in our knowledge of the system and its capabilities in natural environments. Geckos are extremely diverse, constituting more than 1,400 species worldwide (22, 23). However, knowledge of the natural substrates and conditions geckos use is very limited. For example, it is likely that many species move across leaves and other plant structures that are not perfectly smooth and have variable surface chemistries (24,25). In principle, the interaction of gecko feet with such surfaces may have a significant effect on adhesion, yet gecko research has only just begun to tackle such questions (26-28). Additionally, natural surfaces are likely to become wet (especially in the tropics) and dirty, potentially reducing adhesion. Although research on the ability of geckos to remove dirt from their toes has received some attention (29,30), studies on wetting and the effect of water are limited, despite the well-known antiwetting properties of the toes, which are superhydrophobic and have a low-contact-angle hysteresis (31, 32).Somewhat surprisingly, geckos cannot stick to hydrophilic glass when it is covered with a layer of water (33). Anecdotally, this effect has been long and widely appreciated; nevertheless, the effect of water on gecko adhesion is complex. For example, a thin water ...

Adhesive interactions of geckos with wet and dry fluoropolymer substrates

“…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].…”

“…These 'non-stick' surfaces, like commercially available poly(tetrafluoroethylene) (PTFE), have been known to foil the gecko's incredible adhesive system, rendering them virtually dysfunctional (i.e. unable to support their body weight) [8,16,17]. While not many studies have investigated adhesion to wet fluorinated surfaces, the results are intriguing.…”

“…For example, when testing gecko-inspired PTFE pillars, underwater adhesion was 70% of adhesion in air [18]. Yet when testing live geckos on PTFE, shear adhesion is very low in air but substantial in water (ratio of wet to dry adhesion is 5.19) [8]. The enhancement of whole-animal shear adhesion underwater was particularly surprising considering the very low shear adhesion values in air on PTFE (1.6 N in air versus 8 N in water).…”

Advances in Bio-inspired Tribology for Engineering Applications

“…2.2 Gecko: It is not About their Feet Anymore! For over nearly a decade now, gecko's feet exhibiting remarkable adhesion properties has been a significant attraction and inspiration to many tribologists [34][35][36][37]. But this overwhelming interest toward their feet had led to ignorance of other fascinating regions of a lizard's body.…”

“…The gecko skin provides a unique topographical template for multifunctional man-made designs which may potentially aid in areas as diverse as self-cleaning of outdoor and indoor surfaces environments (e.g., hospital surfaces, habitat structures) [46][47][48] and a variety of other applications such as artificial micro-channels and circulatory channels (e.g., syringes, central line ports, next-generation animal capillaries), [49] dental implants, contact lenses, wound-healing architectures, marine structures, and membranes used in industrial applications (e.g., potable water filters) [38,[50][51][52][53][54].…”