Effects of surface wettability on gecko adhesion underwater

Dryden

Olderman

et al. 2015

J. R. Soc. Interface.

Fluorinated substrates like Teflon w (poly(tetrafluoroethylene); PTFE) are well known for their role in creating non-stick surfaces. We showed previously that even geckos, which can stick to most surfaces under a wide variety of conditions, slip on PTFE. Surprisingly, however, geckos can stick reasonably well to PTFE if it is wet. In an effort to explain this effect, we have turned our attention to the role of substrate surface energy and roughness when shear adhesion occurs in media other than air. In this study, we removed the roughness component inherent to commercially available PTFE and tested geckos on relatively smooth wet and dry fluoropolymer substrates. We found that roughness had very little effect on shear adhesion in air or in water and that the level of fluorination was most important for shear adhesion, particularly in air. Surface energy calculations of the two fluorinated substrates and one control substrate using the Tabor-Winterton approximation and the Young-Dupré equation were used to determine the interfacial energy of the substrates. Using these interfacial energies we estimated the ratio of wet and dry normal adhesion for geckos clinging to the three substrates. Consistent with the results for rough PTFE, our predictions show a qualitative trend in shear adhesion based on fluorination, and the quantitative experimental differences highlight the unusually low shear adhesion of geckos on dry smooth fluorinated substrates, which is not captured by surface energy calculations. Our work has implications for bioinspired design of synthetics that can preferentially stick in water but not in air.

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Adhesive interactions of geckos with wet and dry fluoropolymer substrates

Dryden

Olderman

et al. 2015

J. R. Soc. Interface.

“… 18 for a setal structure hypothesis). Furthermore, new properties of the natural system, such as adhesion to wet substrates and self-drying, are still being discovered and clarified 19 20 21 22 23 24 . Finally, although the mechanism behind gecko adhesion has been defined as mainly van der Waals-based 25 , the roles of capillary adhesion, electrostatic charging and material charge, and nanobubbles in the presence of water 20 26 27 28 29 30 , has called our attention to adhesion under the non-ideal conditions that geckos encounter on a routine basis.…”

mentioning

confidence: 99%

The effect of temperature and humidity on adhesion of a gecko-inspired adhesive: implications for the natural system

Klittich

Sitti

et al. 2016

Sci Rep

The adhesive system of geckos has inspired hundreds of synthetic adhesives. While this system has been used relentlessly as a source of inspiration, less work has been done in reverse, where synthetics are used to test questions and hypotheses about the natural system. Here we take such an approach. We tested shear adhesion of a mushroom-tipped synthetic gecko adhesive under conditions that produced perplexing results in the natural adhesive system. Synthetic samples were tested at two temperatures (12 °C and 32 °C) and four different humidity levels (30%, 55%, 70%, and 80% RH). Surprisingly, adhesive performance of the synthetic samples matched that of living geckos, suggesting that uncontrolled parameters in the natural system, such as surface chemistry and material changes, may not be as influential in whole-animal performance as previously thought. There was one difference, however, when comparing natural and synthetic adhesive performance. At 12 °C and 80% RH, adhesion of the synthetic structures was lower than expected based on the natural system’s performance. Our approach highlights a unique opportunity for both biologists and material scientists, where new questions and hypotheses can be fueled by joint comparisons of the natural and synthetic systems, ultimately improving knowledge of both.

“…Second, there is still ongoing debate about the mechanisms of gecko adhesion. Consider the recent study proposing that contact electrification is more important than van der Waals forces in determining gecko adhesion (Izadi et al, 2014), or a second study which suggests that, underwater, nanobubbles (see Glossary) -rather than fundamental thermodynamic contributions arising from two contacting surfaces in water -play a role in adhesion (Peng et al, 2014). Both of these shortcomings in understanding the gecko adhesive system will ultimately limit our ability to use the relevant principles of design and consequent function to create synthetic mimics.…”

Section: Water Water Everywherementioning

confidence: 99%

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

Niewiarowski

Journal of Experimental Biology

Dhinojwala

2016

The natural clinging ability of geckos has inspired hundreds of studies seeking design principles that could be applied to creating synthetic adhesives with the same performance capabilities as the gecko: adhesives that use no glue, are self-cleaning and reusable, and are insensitive to a wide range of surface chemistries and roughness. Important progress has been made, and the basic mechanics of how 'hairy' adhesives work have been faithfully reproduced, advancing theory in surface science and portending diverse practical applications. However, after 15 years, no synthetic mimic can yet perform as well as a gecko and simultaneously meet of all the criteria listed above. Moreover, processes for the production of inexpensive and scalable products are still not clearly in view. Here, we discuss our perspective on some of the gaps in understanding that still remain; these gaps in our knowledge should stimulate us to turn to deeper study of the way in which free-ranging geckos stick to the variety of surfaces found in their natural environments and to a more complete analysis of the materials composing the gecko toe pads.