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Directional water collection of wet spider silk

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Document pages: 12 pages

Abstract: Many biological surfaces in the plant and animal world have unusual structural characteristics on the micron and nano scales, which control their interaction with water and thus their wettability 1,2,3,4,5. Desert beetles provide an interesting example of how they use micron sized hydrophobic and hydrophilic areas on their backs to capture moisture from moist air. Anyone who has seen a spider web decorated with dew will appreciate that spider silk can also effectively collect water from the air. Here, we prove that the water collecting ability of the capture silk of the sieve spider uloborus walkenaerius is the result of the unique fiber structure formed after wetting, and the "wet reconstruction " fiber is characterized by random nnofibrils and separated by joints made of aligned nanofibrils. These structural features result in a surface energy gradient between the spindle-knots and the joints and also in a difference in Laplace pressure, with both factors acting together to achieve continuous condensation and directional collection of water drops around spindle-knots. Submillimetre-sized liquid drops have been driven by surface energy gradients7,8,9 or a difference in Laplace pressure10, but until now neither force on its own has been used to overcome the larger hysteresis effects that make the movement of micrometre-sized drops more difficult. By tapping into both driving forces, spider silk achieves this task. Inspired by this finding, we designed artificial fibres that mimic the structural features of silk and exhibit its directional water-collecting ability.

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