Surface and Wetting Properties of Embiopteran (Webspinner) Nanofiber Silk

Abstract

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.

Structural and wetting properties of nature's finest silks (order Embioptera)

“…ceylonica (Oligotomidae) and S. calypso (Anisembiidae)), were chosen because they share similar habitats, which experience heavy tropical rains, a situation that potentially imposes selective pressures on the insects to produce effective silk coverings. A webspinner silk fibre is thought to be composed of a lipid coating surrounding a protein core [ 5 , 6 ]. Without this lipid coating, the silk fibre was observed with TEM to be permeable to water [ 6 ].…”

“…A webspinner silk fibre is thought to be composed of a lipid coating surrounding a protein core [ 5 , 6 ]. Without this lipid coating, the silk fibre was observed with TEM to be permeable to water [ 6 ]. Previous NMR studies described how organic solvents (2 : 1 chloroform/methanol) changed the hydrophobicity and permeation of webspinner silks to water [ 5 , 6 ].…”

“…Without this lipid coating, the silk fibre was observed with TEM to be permeable to water [ 6 ]. Previous NMR studies described how organic solvents (2 : 1 chloroform/methanol) changed the hydrophobicity and permeation of webspinner silks to water [ 5 , 6 ]. In the current study, we evaluate whether in the absence of organic solvents, exposure of webspinner silk to water can also change its water permeability properties and macroscopic structure.…”

“…urichi silk included contact angle measurements, which indicated that the silk was superhydrophobic (water drops bead up on it) but also highly water adhesive (water drops do not roll off) [ 6 ]. Silken sheets were composed of sparse, unevenly spaced fibres, which were laid ‘down in layers’ and not uniform in thickness [ 6 ]. High advancing contact angles of 150° ± 2° and low receding contact angles (approaching 0°) were measured [ 6 ].…”

“…Silken sheets were composed of sparse, unevenly spaced fibres, which were laid ‘down in layers’ and not uniform in thickness [ 6 ]. High advancing contact angles of 150° ± 2° and low receding contact angles (approaching 0°) were measured [ 6 ]. High contact angle hysteresis and adhesive quality have also been previously quantified for the surfaces of rose petals [ 14 , 15 ].…”