JEB desktop wallpaper calendar 2016

JEB desktop wallpaper calendar 2016

Changes in materials properties explain the effects of humidity on gecko adhesion
Jonathan B. Puthoff, Michael S. Prowse, Matt Wilkinson, Kellar Autumn

SUMMARY

Geckos owe their remarkable stickiness to millions of dry setae on their toes, and the mechanism of adhesion in gecko setae has been the topic of scientific scrutiny for over two centuries. Previously, we demonstrated that van der Waals forces are sufficient for strong adhesion and friction in gecko setae, and that water-based capillary adhesion is not required. However, recent studies demonstrated that adhesion increases with relative humidity (RH) and proposed that surface hydration and capillary water bridge formation is important or even necessary. In this study, we confirmed a significant effect of RH on gecko adhesion, but rejected the capillary adhesion hypothesis. While contact forces of isolated tokay gecko setal arrays increased with humidity, the increase was similar on hydrophobic and hydrophilic surfaces, inconsistent with a capillary mechanism. Contact forces increased with RH even at high shear rates, where capillary bridge formation is too slow to affect adhesion. How then can a humidity-related increase in adhesion and friction be explained? The effect of RH on the mechanical properties of setal β-keratin has escaped consideration until now. We discovered that an increase in RH softens setae and increases viscoelastic damping, which increases adhesion. Changes in setal materials properties, not capillary forces, fully explain humidity-enhanced adhesion, and van der Waals forces remain the only empirically supported mechanism of adhesion in geckos.

FOOTNOTES

  • This research was supported by the National Science Foundation under awards NBM 0900723 and IOS 0847953.

  • LIST OF SYMBOLS AND ABBREVIATIONS

    A
    cross-sectional area
    c1, c2, c3
    non-linear fit parameters for (E′)–1
    DMA
    dynamic mechanical analysis
    E, E′, E
    complex modulus, storage modulus, loss modulus
    E*
    contact modulus
    F, Fa, Ff
    applied force, adhesion force, frictional force
    F0
    substrate-dependent factor in Fa
    GSA
    gecko-like synthetic adhesive
    m, ζ
    phenomenological exponents
    o
    asymptotically small terms in Fa
    R
    fiber radius
    RH
    relative humidity
    t
    time
    tcap
    capillary bridge formation time
    v
    drag velocity
    vc
    crack extension velocity
    vdW
    ‘van der Waals’
    α
    coefficient of highest order term in Fa
    γ, γ0, Δγ
    work of adhesion, surface energy, viscoelastic contribution to work of adhesion
    δ
    loss angle
    ϵ(t), ϵ0
    dynamic strain, strain amplitude
    ζGaAs, ζSiO2
    empirical exponents
    θ
    contact angle
    μ
    coefficient of friction
    ν
    Poisson's ratio
    σ, σa
    stress applied to fiber, fiber pull-off stress
    σ(t), σ0
    dynamic stress, stress amplitude
    τ
    spatula slip timescale
    Φ
    multiplicative viscoelastic factor in work of adhesion
    ω
    frequency
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