By Janet Fang
Researchers have puzzled out how spider silk is able to catch the morning dew. Their findings may lead to the development of new materials that are able to capture water from the air.
The study, published today in Nature, examines the silk of the hackled orbweaver spider Uloborus walckenaerius. "Bright, pearl-like water drops hang on thin spider silk in the morning after fogging," says study author Lei Jiang from the Beijing National Laboratory for Molecular Sciences. "It is unexpected and interesting. Human hair can't do that."
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Dry spider silk forms a necklace-like structure. Two main fibers support a series of separate rounded "puffs," each made up of tiny, randomly intertwined nanofibrils. When water vapor condenses onto these puffs, they shrink into densely packed knots, shaped like spindles (or two cones with their bases stuck together). Thinner connecting stretches of nanofibrils, separating the knots, become more apparent; these areas are called "joints."
The researchers studied the webs under both electron and light microscopes. They noticed that as water condenses on the web, droplets move towards the nearest spindle-knot, where they coalesce to form larger drops.
The spindle-knots have a rough surface, because the fibrils within them are randomly interweaved. But the joints between the knots have a smooth texture, because their constituent fibrils run parallel to each other. It is this difference in roughness that helps to water drops to slide towards the spindle-knots, sticking when they arrive.
The cone shape of the spindle-knots also drives droplets towards their centre. Once they hit the edge of a cone, drops are propelled towards its base, the least curved region, because of the pressure difference caused by surface tension.
Mimicking nature
Guided by their findings, the team made their own artificial spider silk using nylon fibers dipped in a polymer solution that, when dried, formed spindle-knots similar to those in natural spider silk. They anticipate that their studies of these fibers could lead to new materials for collecting water from the air.
"It is impressive that they were able to produce an analogue of wetted [spider] thread that duplicated the properties that they observed," says spider silk expert Brent Opell of Virginia Tech in Blacksburg.
But it doesn't seem likely that natural selection has directed the evolution of this particular spider's silk for water collection, he adds. The spider's thread seems to have evolved to work best when it is dry.
As Jiang and his colleagues show, when the spider silk is wetted, the fibrils are matted down. "From a spider's perspective, this is a bad thing because it reduces the web's ability to capture prey," Opell says.
"The authors of this paper are studying an artefact," says zoologist and spider-silk expert Fritz Vollrath of the University of Oxford, UK, "which is still interesting although it has no biological function".
