Teflon hierarchical nanopillars with dry and wet adhesive properties

Izadi, Hadi; Zhao, Boxin; Han, Yougun; McManus, Neil T.; Penlidis, Alexander

doi:10.1002/polb.23076

Cited by 27 publications

(50 citation statements)

References 28 publications

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“…It is clear that further work is necessary to clarify the effect of roughness on adhesion to wet and dry surfaces. Interestingly, synthetic gecko-like PTFE pillars tested underwater with a silica probe also were successful in achieving adhesion; however, adhesion values underwater were not five times higher than dry as measured in our experiments (48).…”

Section: Discussioncontrasting

confidence: 43%

Surface wettability plays a significant role in gecko adhesion underwater

Stark

Badge

Wucinich

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

126

192

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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 ...

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Section: Discussioncontrasting

confidence: 43%

Surface wettability plays a significant role in gecko adhesion underwater

Stark

Badge

Wucinich

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

126

192

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“…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].…”

Section: Introductionmentioning

confidence: 99%

Adhesive interactions of geckos with wet and dry fluoropolymer substrates

Stark

Dryden

Olderman

et al. 2015

J. R. Soc. Interface.

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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.

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“…PSAs are usually made of a viscoelastic polymer. To improve adhesion, low stiffness polymers are used, e.g., [8,9,[17][18][19][20]. Although choosing a soft polymer helps the adhesive to strongly attach to the surface, the adhesive may become so weak to tolerate large vdW forces which may cause rupture of the fibers during detachment or may result in formation of clumps when fibers approach each other.…”

Section: Introductionmentioning

confidence: 99%

“…6 and obtain the critical length for additional comparisons with the outcome of Eq. (20). To continue with simulations, a Hamaker constant of 51 Â 10 À21 J [36] and a Poisson's ratio of 0.45 were chosen.…”

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A New Model for Predicting Fiber Clumping Phenomenon in Bio-Inspired Dry Adhesives

Ahmadi

Menon

2013

The Journal of Adhesion

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Self-adhesion, or clumping, primarily occurs in bio-inspired dryadhesives when two or more adjacent fibers become in contact and adhere to each other due to large van der Waals (vdW) intermolecular forces. Studying self-adhesion is of primary interest as dry-adhesives drastically lose their adhesion strength after few load cycles if clumping occurs. An optimization procedure to select material properties, shape, and geometry of the fibers is therefore highly desirable for both improving the adhesion and reducing the probability of clumping. Few numerical models have been utilized to attain optimal geometry and material parameters. These models are developed based on the small-deflection beam theory and use an over-simplified elastic-contact model to determine adhesion forces. In this work, a model suitable to simulate large deformations with distributed intermolecular forces is proposed. The model is based on the finite element method (FEM) and uses an adaptive meshing procedure to accurately calculate adhesion forces. By using this model, clumping is studied and predictions are compared against experimental results obtained from the literature. In addition to the FEM model, an analytical model that considers large-deflection reactions of fibers is proposed to corroborate finite element results. Results from analytical and finite element models show precise clumping predictions.

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Teflon hierarchical nanopillars with dry and wet adhesive properties

Cited by 27 publications

References 28 publications

Surface wettability plays a significant role in gecko adhesion underwater

Surface wettability plays a significant role in gecko adhesion underwater

Adhesive interactions of geckos with wet and dry fluoropolymer substrates

A New Model for Predicting Fiber Clumping Phenomenon in Bio-Inspired Dry Adhesives

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