Surface and Wetting Properties of Embiopteran (Webspinner) Nanofiber Silk

Insects of the order
Embioptera, known as embiopterans, embiids,
or webspinners, weave silk fibers together into sheets to make shelters
called galleries. In this study, we show that silk galleries produced
by the embiopteran Antipaluria urichi exhibit a highly hydrophobic wetting state with high water adhesion
macroscopically equivalent to the rose petal effect. Specifically,
the silk sheets have advancing contact angles above 150°, but
receding contact angle approaching 0°. The silk sheets consist
of layered fiber bundles with single strands spaced by microscale
gaps. Scanning and transmission electron microscopy (SEM, TEM) images
of silk treated with organic solvent and gas chromatography mass spectrometry
(GC-MS) of the organic extract support the presence of a lipid outer
layer on the silk fibers. We use cryogenic SEM to demonstrate that
water drops reside on only the first layer of the silk fibers. The
area fraction of this sparse outer silk layers is 0.1 to 0.3, which
according to the Cassie–Baxter equation yields an effective
static contact angle of ∼130° even for a mildly hydrophobic
lipid coating. Using high magnification optical imaging of the three
phase contact line of a water droplet receding from the silk sheet,
we show that the high adhesion of the drop stems from water pinning
along bundles of multiple silk fibers. The bundles likely form when
the drop contact line is pinned on individual fibers and pulls them
together as it recedes. The dynamic reorganization of the silk sheets
during the droplet movement leads to formation of “super-pinning
sites” that give embiopteran silk one of the strongest adhesions
to water of any natural hydrophobic surface.