Hooked on Phononics: Spider Silk Sound and Heat

      New discoveries about spider silk could inspire new materials to manipulate heat and sound in the same way semiconducting circuits manipulate electrons, according to scientists at Rice University.

      A paper published yesterday in Nature Materials looks at the microscopic structure of spider silk and reveals unique characteristics in the way it transmits phonons, quasiparticles of sound.

      A phonon (cool moving graphic at this link) is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter, like solids and some liquids. It represents an excited state in the quantum mechanical quantization of the modes of vibrations of elastic structures of interacting particles.

     The research shows for the first time that spider silk has a phonon band gap. That means it can block phonon waves in certain frequencies in the same way an electronic band gap -- the basic property of semiconducting materials -- allows some electrons to pass and stops others.

     The researchers wrote that their observation is the first discovery of a "hypersonic phononic band gap in a biological material."

      How the spider uses this property remains to be understood, but there are clear implications for materials, according to materials scientist and Rice Engineering Dean Edwin Thomas, who co-authored the paper. He suggested that the crystalline microstructure of spider silk might be replicated in other polymers. That could enable tunable, dynamic metamaterials like phonon waveguides and novel sound or thermal insulation, since heat propagates through solids via phonons.

     "Phonons are mechanical waves," Thomas said, "and if a material has regions of different elastic modulus and density, then the waves sense that and do what waves do: They scatter. The details of the scattering depend on the arrangement and mechanical couplings of the different regions within the material that they're scattering from."

       Spiders are adept at sending and reading vibrations in a web, using them to locate defects and to know when "food" comes calling. Accordingly, the silk has the ability to transmit a wide range of sounds that scientists think the spider can interpret in various ways. But the researchers found silk also has the ability to dampen some sound.

      "(Spider) silk has a lot of different, interesting microstructures, and our group found we could control the position of the band gap by changing the strain in the silk fiber," Thomas said. "There's a range of frequencies that are not allowed to propagate. If you broadcast sound at a particular frequency, it won't go into the material."

     Thomas and other researchers decided to take a more detailed look at dragline silk, shown below in a SEM, which spiders use to construct a web's outer rim and spokes and as a lifeline. (A spider suspended in midair is clinging to a dragline.) Though silk has been studied for thousands of years, it has only recently been analyzed for its acoustic properties.

      "Silk is a hierarchical structure comprised of a protein, which folds into sheets and forms crystals. These hard protein crystals are interconnected by softer, amorphous chains," Thomas said. Stretching or relaxing the interconnecting chains changes the silk's acoustic properties by adjusting the mechanical coupling between the crystals.

       "Right now, we don't know how to do any of this in other macromolecular fiber materials," Thomas said. "There's been a fair amount of investigation on synthetic polymers like nylon, but nobody's ever found a band gap."

Have you ever found a band gap?!
Steph

Zoë and her "Camp English" group in Ethiopia this summer: