February 26, 2010 | 1
Engineers continue to tinker with plastics and chemical coatings for use in products designed to stay dry (or keep their wearers dry), but nature solved the problem of water resistance a long time ago. The leaves of plants such as lotus and nasturtium, for instance, make water bead up like rain on a freshly waxed car, and the African Stenocara beetle uses water-repellent channels on its back to funnel water droplets into its mouth.
Following in the footsteps of researchers who have sought to engineer bio-inspired materials that harness the water-repelling, or hydrophobic, properties of lotus leaves and Stenocara beetles, a new paper proposes a relatively simple method to mimic the hydrophobicity of arthropod hairs. Some invertebrates of the phylum Arthropoda, which includes insects, crustaceans and spiders, have fuzzy surfaces that allow them to walk on water, as is the case with water striders, or to trap air bubbles for underwater respiration—a sort of rudimentary scuba apparatus.
Shu-Hau Hsu and Wolfgang Sigmund of the University of Florida report in the February 2 issue of Langmuir that they created almost perfectly hydrophobic surfaces by melting polypropylene sheets and pressing them against molds dotted with tiny pores. (Hydrophobicity is measured by how much a water droplet contacts the surface; on a perfectly hydrophobic material water will bead into spheres that make minimal contact.) When the mold was removed, the material that had filled the pores formed a network of hair-like appendages [see micrograph below] rising above the surface of the polypropylene sheet in a "Γ" shape. The fuzzy material resembles the hair-covered surfaces found on arthropods such as the water walkers of the genus Mesovelia.
Such structures limit water contact by trapping air pockets underneath the droplet, which reduces the amount of contact between the liquid and the surface. With the smallest artificial hairs Hsu and Sigmund created, just 0.6 micron in diameter, the contact angle of the water droplet and the surface was greater than 170 degrees. (A micron is one millionth of a meter.) On perfectly hydrophobic materials, spherical droplets contact the surface at a 180-degree angle.
One benefit to Hsu and Sigmund’s approach is that the material’s hydrophobicity is purely structural—that is, it does not require an additional chemical treatment with a hydrophobic agent. (Flat polypropylene is itself only very weakly hydrophobic.) But the authors note that their hairs are not very robust; rubbing the surfaces with one’s fingers flattens the forest of fuzz and destroys its superb hydrophobicity.
Water beads on an artificial-hair surface (above) and scanning-electron microscope image of 0.6-micron hairs (below): © 2010 The Sigmund Group