Elastocapillary windlass: from spider silk to smart actuators

Abstract : For more details about my past and current research projects, find me at herveelettro.wordpress.com. The goal of this PhD thesis was to understand and recreate artificially a self-assembling mechanism involving capillarity and elasticity present in natural spider silk. The primary function of the micronic glue droplets that exist on spider capture silk is to provide the spider web with adhesive properties, crucial in attaining efficiency as a food trap. These droplets play yet another role: the dramatic enhancement of silk mechanical properties, as well as the preservation of the integrity of the web structure. This is due to the localization of the buckling instability within the liquid glue droplets, site of over-compression due to the capillary meniscii. This leads to local coiling of the fibre, and retightening of the overall system. In effect, this is a micronic automatic coiling system that is powered by capillarity, and is thus coined elasto-capillary windlass. The first part of my thesis aimed to the characterization of natural samples and visualization of the natural windlass. This required adjustements of environmental parameters (especially relative humidity), microscopic observations and sub-micronewton force measurements both in compression and in tension, as well as image analysis and technical problem solving. We have found that the local shape of the fibre is intimately linked to the mechanical properties of the overall sample. The existence of the windlass mechanism implies that under compression this special drop-on-fibre system behaves like a liquid, whereas under tension it has a classical elastic spring regime. Spiders have thus found a way to create liquid-solid mechanical hybrids using shape-induced functionalisation. We use a fully mechanical model to explain this unique behaviour, as well as an analogy with phase transition formalism. Using a drop-on-deformable-fibre system, we show that if the wetting energy is higher than the bending energy, the system “activates” and in-drop coiling begins. Numerical simulations of 3D elastica under local soft confinement potential reproduce the observed link between local fibre shape and mechanical response. We then fabricate centimeter-long micronic soft fibres, by melt spinning or wet spinning of thermoplastic polymers, and show that the simple addition of a wetting liquid droplet makes for an effective system with mechanical properties quantitatively close to that of spider capture silk. Further experimental characterization of the created drop-on-coilable-fibre systems was found to agree with predictions from numerical simulations and theory, especially for properties such as the threshold for activation, the existence of an hysteresis, the fine details of the stress-strain curve, or the influence of gravity and of the deformability of the droplet interface. We further showed that the drop-on-coilable-fibre systems can be enriched by the addition of new degrees of freedom, such as using several fibres, temperature changes or evaporation. This led to the design of actuators and sensors, as well as a new technique for 3D microfabrication, showing the potential of increasingly complex cases of drop-on-coilable-fibre systems, both technologically and academically.
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https://hal.sorbonne-universite.fr/tel-01200518
Contributor : Hervé Elettro <>
Submitted on : Friday, December 4, 2015 - 5:07:22 PM
Last modification on : Thursday, March 21, 2019 - 12:59:20 PM
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  • HAL Id : tel-01200518, version 2

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Hervé Elettro. Elastocapillary windlass: from spider silk to smart actuators. Mechanics of the fluids [physics.class-ph]. UPMC, 2015. English. ⟨tel-01200518v2⟩

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