Elasto-capillary windlass: from spider silk to smart actuators

Inspired by spider silk, we showed that it is possible to functionalize a simple fiber into a solid-liquid mechanical hybrid simply by combining it with a droplet. In the right conditions, the fiber may be coiled within the drop’s cavity by capillary forces, like a micro-winch. The core fiber bestows resistance under tension on the so called drop-on-coilable-fiber system. But under compression, the drop stores the spare fiber and keep a state of tension, as seen in this video. These new hybrid systems thus combine both the mechanical advantages of solids and liquids. This could lead to new technological breakthroughs in terms of flexible electronics, reconfigurable microsystems and highly reversible self-tensioned motors.

Focus article in Science

These results have been published in PNAS (Proceedings of the National Academy of Science), in open access here. It received significant attention from the media, including:Capture d’écran 2017-05-16 à 16.48.50

Related publications:

  1. Influence of a gravitational field on drop-on-coilable-fiber systems

    H. Elettro, S. Neukirch & A. Antkowiak

    In preparation (2017)

  2. Damage control through windlass mechanism in ecribellate webs

    H. Elettro, F. Vollrath, A. Antkowiak & S. Neukirch

    In preparation (2017)

  3. Drop-on-coilable-fibre systems exhibit negative stiffness events and transitions in coiling morphology

    H. Elettro, S. Neukirch, F. Vollrath & A. Antkowiak

    Soft Matter (In Press, 2017)

    Open Access PDF FileFront Cover

  4. In-drop capillary spooling of spider capture thread inspires hybrid fibers with mixed solid–liquid mechanical properties

    H. Elettro, S. Neukirch, A.Antkowiak & F. Vollrath

    Proceedings of the National Academy of Science (PNAS), 113(22):6143-6147 (2016)

    Open access PDF FileEditor’s suggestion

  5. Elastocapillary coiling of an elastic rod inside a drop

    H. Elettro, P. Grandgeorge, S. Neukirch

    Journal of Elasticity, 127(2):235-247 (2016)

    PDF File

  6. Coiling of an elastic beam inside a disk: A model for spider-capture silk

    H. Elettro, S. Neukirch, A.Antkowiak & F. Vollrath

    International Journal of Non-Linear Mechanics, 75:59–66 (2015)

    PDF File

  7. Adhesion of dry and wet cribellate silk

    H. Elettro, S. Neukirch, A.Antkowiak & F. Vollrath

    The Science Of Nature, 102(7):1–4 (2015)

    PDF File

The following scientific results represent my 3-years PhD project, performed under the supervision of Prof. Neukirch and Prof. Antkowiak at University Pierre and Marie Curie (2012-2015, Paris, France). Here we observe for the first time a self-assembling mechanism involving capillarity and elasticity in natural samples of spider silk. This biomaterial has been proved to be a astonishing resource for innovation in material scienceadhesive technology and even fiber optics.

The primary function of the micronic glue droplets that exist on spider capture silk (see figure 1.a) 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 fiber, 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.

Figure 1- a. Microscopic view of a spider capture silk thread from Nephila Madagascariensis (right, from David Monniaux). The core silk thread is covered with micronic glue droplets. b. Optical micrograph view of the elastocapillary coiling of natural spider silk filament within one of the glue droplets. The capillary overcompression localizes the buckling instability and retightens  the entire system, leading to liquid-like behavior under compression. c. PolyUrethane microfibers and silicone oil droplets present the same unusual mechanical response than spider silk.

The first part of this thesis aimed to the characterization of natural samples and visualization of the natural windlass (see figure 1.b). This required adjustements of environmental parameters (especially relative humidity), microscopic observations and nanonewton force measurements both in compression and in tension, as well as image analysis and technical problem solving. We then fabricated centimeter-long micronic soft fibers, by melt spinning or wet spinning of various thermoplastic polymers (see figure 1.c), and showed that the simple addition of a wetting liquid droplet produces a system with mechanical properties quantitatively close to that of spider capture silk.

Shape-induced functionalization, fluid-solid mechanical hybrid

We have found that the local shape of the fiber 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-fiber system behaves like a liquid, whereas under tension it presents a classical elastic spring behavior (see figure 2 right). Spiders have thus found a way to build liquid-solid mechanical hybrids using shape-induced functionalization.

We used a fully mechanical model to explain this unique behaviour, as well as an analogy with phase transition formalism. Using a drop-on-deformable-fiber system, We showed 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 fiber shape and mechanical response (see figure 2 right).

Further experimental characterization of the created drop-on-coilable-fiber systems was found to agree with predictions from numerical simulations and theory, especially for properties such as the threshold for activation (see figure 2 left), 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.

Figure 2- Left: Experiments on various materials and liquids show that the elastocapillary coiling mechanism is universal, and thus can be obtained with seemingly any material. For in-drop coilability, the capillary force must simply be higher than the local buckling threshold (dashed line). Right: 3D numerical simulations (black line and insets) are found to agree quantitatively very well with experimental nanonewton force measurements (green line). Both reveal the fine structure of the unconventional mechanical response of drop-on-coilable-fiber systems, that behave as solids in tension and as liquids in compression. This curious switching in mechanical behavior happens through a counter-intuitive negative stiffness transition regime.

We further showed that the drop-on-coilable-fiber systems can be enriched by the addition of new degrees of freedom, such as using several fibers, 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-fiber systems, both technologically and academically.