title: 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.
authors.affiliation: School of Molecular Sciences, Arizona State University , Tempe, Arizona 85287-1604, United States.
authors.forename: Thomas M
authors.initials: TM
authors.lastname: Osborn Popp
authors.affiliation: School of Molecular Sciences, Arizona State University , Tempe, Arizona 85287-1604, United States.
authors.forename: J Bennett
authors.initials: JB
authors.lastname: Addison
authors.affiliation: School of Molecular Sciences, Arizona State University , Tempe, Arizona 85287-1604, United States.
authors.forename: Jacob S
authors.initials: JS
authors.lastname: Jordan
authors.affiliation: School for Engineering of Matter, Transport and Energy, Arizona State University , Tempe, Arizona 85287-1604, United States.
authors.forename: Viraj G
authors.initials: VG
authors.lastname: Damle
authors.affiliation: School for Engineering of Matter, Transport and Energy, Arizona State University , Tempe, Arizona 85287-1604, United States.
authors.forename: Konrad
authors.initials: K
authors.lastname: Rykaczewski
authors.affiliation: LeRoy Eyring Center for Solid State Science, Arizona State University , Tempe, Arizona 85287-1604, United States.
authors.forename: Shery L Y
authors.initials: SL
authors.lastname: Chang
authors.affiliation: Department of Chemistry and Biochemistry, Santa Clara University , Santa Clara, California 95053, United States.
authors.forename: Grace Y
authors.initials: GY
authors.lastname: Stokes
authors.affiliation: Department of Biology, Santa Clara University , Santa Clara, California 95053, United States.
authors.forename: Janice S
authors.initials: JS
authors.lastname: Edgerly
authors.affiliation: School of Molecular Sciences, Arizona State University , Tempe, Arizona 85287-1604, United States.
authors.forename: Jeffery L
authors.initials: JL
authors.lastname: Yarger
mesh_terms: D000818:Animals; D055595:Mechanical Phenomena; D057139:Nanofibers / Q000737:chemistry*; D000074746:Neoptera; D047011:Silk / Q000737:chemistry*; D014867:Water / Q000737:chemistry; D017155:Wettability
chemical_list: D047011:Silk; D014867:Water