Add a drop of water to a strip of this paper-like material and something astounding happens: It clamps shut like a clam. In a split second the liquid has turned the material into a tiny tube, like a self-rolling cigarette.

This odd new material, called a Janus bilayer, was created by a team of materials scientists led by William Wong at the Australian National University in Canberra. Made from a paper-thin sandwich of two material layers, the bilayer acts a lot like the sensitive plant (Mimosa pudica) which protectively folds itself at slightest touch, even the thump of a falling raindrop. The material is described today in the journal Science Advances.

"If you put a water droplet on one of the end of our material... it folds around the water droplet and at the same time propels some of the water further, causing a chain of folding-water propulsion reactions," says Zuankai Wang, a mechanical engineer with the team at the City University of Hong Kong. "As the water its transported during the re-shaping the folding stimulus can propagate for theoretically unlimited distances resulting in very large structures."

Water Wrapping

The secret of the self-folding trick lies in the combination of the two layered materials, each of which responds differently to water. On the bottom of the material is a razor-thin and stretchy layer of PVC, the same plastic material used to create a plumber's PVC pipe. This "serves as a water-impenetrable backbone," writes William Wong in the study. On top of that elastic PVC is a sheet of water-absorbing polyester called polycaprolactone. There's no glue here. The two materials cling together via the electrostatic charge called the Van der Waals force.

Wong et. al.

When a drop of water lands on the Janus bilayer, the liquid hydrates the the polyester, shrinking it. That rapid shrinking causes the entire material to roll in on itself and curl up tight. This initial reaction takes only 33 miliseconds. But because the water can't seep through the PVC and out of the material, it's forced further down the material, causing a cascading chain reaction. More of the material rolls shut, pushing the water down further and further.

Later, the scientists can simply unfold the material by evaporating or otherwise extracting the water. This can be quickly done by dipping the material into a bath of ethanol. Bam, good as new.

Smart Clothes?

Wang says that with some tweaks, a future version of this material could be integrated into a weather-respondent clothing. "You could imagine a material that becomes water-proof if slightly wet, but would otherwise be very porous and enable free evaporation from your skin," he says.

Wang's hope for this material does not stop there. "We envision that it can be utilized for numerous wearable and portable applications that do not require power or have significantly reduced power consumption. This includes [uses from] wearable bio-sensors capable of collecting and analyzing sweat and other body fluids" to "fog harvesting for water recollection," Wang says.

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