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Article Dans Une Revue Proceedings of the National Academy of Sciences of the United States of America Année : 2016

In-drop capillary spooling of spider capture thread inspires hybrid fibres with mixed solid-liquid mechanical properties

Résumé

An essential element in the web-trap architecture, the capture silk spun by ecribellate orb spiders consists of glue droplets sitting astride a silk filament. Mechanically this thread presents a mixed solid/liquid behaviour unknown to date. Under extension, capture silk behaves as a particularly stretchy solid, owing to its molecular nanosprings, but it totally switches behaviour in compression to now become liquid-like: it shrinks with no apparent limit while exerting a constant tension. Here, we unravel the physics underpinning the unique behaviour of this " liquid wire " and demonstrate that its mechanical response originates in the shape-switching of the silk filament induced by buckling within the droplets. Learning from this natural example of geometry and mechanics, we manufactured novel programmable liquid wires that present novel pathways for the design of new hybrid solid-liquid materials. H ybrids made of different materials often display effective properties far exceeding those of their components (1): zinc-coated steel is both strong and corrosion-resistant, metal foams (hybrids of metal and air) are stiff, light and crush-able at the same time, making them perfect candidates to absorb energy in a car crash (2, 3). Nature also provides many exquisite examples of hybrid design such as the seashell nacre, both stiff and tough thanks to its inner 'brick-and-mortar' structure composed of rigid, though brittle, inclusions surrounded by a crack arresting soft organic matrix (4), or the bamboo stem with its hollow core and honeycomb-shaped cells that maximize the ratio of bending rigidity over weight (5). A most interesting natural hybrid material is the spi-der's capture thread, which consists of a core filament that supports glue droplets. Here we report on the arresting mechanical behaviour of this capture thread, that changes from solid-like in extension to liquid-like in compression. We trace this behaviour back to the core filament's buckling inside the droplets. A synthetic version of this natural system then allows us to copy the remarkable properties of spider's capture thread to a novel type of hybrid material. Spiders use different kinds of silk to build their webs, and a typical ecribellate orb-web combines dry and smooth radial threads with wet and droplet-covered spiral threads (6, 7, 8, 9, 10). The adhesive nature of these droplets enables the spiral capture thread to perform its primary function of catching insect preys (6). Apart from being sticky, these capture threads also prove to be particularly resilient to tensile tests: extensive studies on their mechanical behaviour (11, 6) revealed that, when stretched, the thread elongates to three times its web-length without breaking and recoils back with no noticeable hysteresis or sagging when relaxed (12). This stretchiness confers spider silk a strength tenfold that of natural or synthetic rubber (13, 14). These remarkable extensional properties rely on the macromolecular architecture of capture silk (15, 16). The ability to cope with stretch is crucial for spider capture threads for it provides their unusually large toughness (energy required for rupture), which in turn allows them to absorb the kinetic energy of incident preys without breaking. Far less understood is the behaviour of the thread when compressed: unlike any solid fibre that sags or buckles, it keeps taut and self-adapts to compression. Figure 1 illustrates this singular behaviour, reminiscent of the response of liquid films to compression events: liquid films do not buckle upon squeezing, but rather self-adapt (17). And as for liquid films, self-adaptation for the capture thread is an indication for fibre self-tension. This liquid-like behaviour in compression suggests that more than merely endowing the web with adhesion, capture silk might well have the additional mechanical function of preserving the web structural integrity. Indeed, without self-adaptation, single sticky strands would touch during relaxation events and thereby irremediably damage the web. With sagging suppressed, the sticky strands are secured apart.
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hal-01316973 , version 1 (19-05-2016)

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Hervé Elettro, Sébastien Neukirch, Fritz Vollrath, Arnaud Antkowiak. In-drop capillary spooling of spider capture thread inspires hybrid fibres with mixed solid-liquid mechanical properties. Proceedings of the National Academy of Sciences of the United States of America, 2016, 13 (22), pp.6143-6147. ⟨10.1073/pnas.1602451113⟩. ⟨hal-01316973⟩
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