Many biological surfaces in both the plant and animal kingdom
possess unusual structural features at the micro- and nanometrescale
that control their interaction with water and hence
wettability1–5. An intriguing example is provided by desert beetles,
which use micrometre-sized patterns of hydrophobic and hydrophilic
regions on their backs to capture water from humid air6. As
anyone who has admired spider webs adorned with dew drops will
appreciate, spider silk is also capable of efficiently collecting water
from air. Here we show that the water-collecting ability of the
capture silk of the cribellate spider Uloborus walckenaerius is
the result of a unique fibre structure that forms after wetting, with
the ‘wet-rebuilt’ fibres characterized by periodic spindle-knots
made of random nanofibrils 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 gradients7–9 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 watercollecting
ability.
