A self-folding straw has been manufactured that turns from a flat strip to a cylinder on encountering water. Besides potentially producing a new line in “just add water” origami, the concept could be used for sampling liquids in potentially toxic environments.
Mimosa pudica is a plant native to South America whose leaves fold up when touched. Artificial materials have been produced that can do something similar on a very small scale, but Australian National University postgraduate student William Wong wanted to expand the scale of operation.
In Science Advances, Wong has announced that stacking a layer of polycaprolactone nanofibers on polyvinyl chloride microfibers creates a material he calls Janus for the opposing behaviors of the two layers.
The polycaprolactone is attracted to water, while the polyvinyl chloride repels water. The attraction of the top layer causes Janus to wrap around any water it encounters, while the hydrophobic underside prevents the water from escaping.
“If we place a drop of water at one end, the material will curl into a tube, delivering the liquid up to 15 centimeters away,” Wong said in a statement. The process is reversible using ethanol, and has the potential to lead to far more elaborate constructions.
“This enables a rapid self-assembly of complex shapes, such as bent, curved and splitting channels, that are a building block for micro-fluidic systems,” Wong said.
Previous self-folding materials, Wong told IFLScience, occurred “on the millimeter scale.” To go beyond the 15 centimeters (6 inches) Wong has achieved will be a challenge. “We are limited by capillary length. You would need a specialized fluid with properties beyond what we have,” he said. “It might be possible in unusual circumstances, such as if you had a huge droplet in space.” One for you, Commander Hadfield.
The work is also ground-breaking in the speed at which the water moves, up to 8 centimeters per second (3 inches per second). A volume of 4.7 microliters per second doesn't sound like much, but it is up to 100 times faster than traditional wicking-based devices.
Wong and his team have shown that water can be made to split at T-junctions and bend around curves. “The potential complexity is almost limitless with laser cutters and craft cutters,” he told IFLScience.
Janus is non-toxic enough to be used in disease sensors, and Wang said it might be used to draw liquids away from an environment too reactive to expose humans to, allowing for testing at a safe distance.
William Wong examines a straw holding water it has folded around. Australian National University
