Scientists at Cornell University have invented a palm-sized device that uses water surface tension as an adhesive bond--an epoxy that can be instantly turned off with the flip of a switch--that they think eventually could be strong enough to hold a human.
Equally if not more important, they believe they also eventually can turn these same “grabbers” into “pushers” with a few additional changes, raising the possibility of far-reaching uses from both functions, from clothing to lifesaving.
The rapid adhesion mechanism could lead to shoes or gloves that stick and un-stick to walls, or Post-it-like notes that can bear loads. The “pushing” feature could be used to open up holes or crevices, or even blocked doors, such as in firefighting.
“You could have materials that can change their properties with a flip of a switch,” said Paul Steen, professor of chemical and biomolecular engineering, who invented the device with Michael Vogel, a former postdoctoral associate. “Right now, they act as ‘grabbers.’ But if you make them into water balloons, then they become ‘punchers,’ or ‘pushers,’ that can push against something else, rather than grab.”
The device was inspired by a beetle native to Florida “with a tenacious ability to defend itself,” Steen said. “The beetle is attacked by ants, and the ants try to pry the beetle loose from a palm leaf and take it back for processing. This bug is the size of a tiny fingernail, but has this dramatic ability to withstand forces by putting down 10,000 tiny liquid contacts.”
Put another way, the beetle can adhere to a leaf with a force 100 times its own weight. It also can instantly un-stick itself.
The device consists of a flat plate patterned with holes, each on the order of microns, or one-millionth of a meter. A bottom plate holds a liquid reservoir, with another porous layer in the middle. An electric field powered by a standard 9-volt battery pumps water through the device, causing droplets to squeeze through the top layer. The surface tension of the exposed droplets makes the device grip another surface, much as two wet glass slides stick together.
“In our everyday experience, these forces are relatively weak,” Steen said. “But if you make a lot of them and can control them, like the beetle does, you can get strong adhesion forces. Each little droplet pushes up against what it wants to grab, and forms a bridge. It tugs. If you have tens of thousands of these droplets, you get a huge tug.”
For example, one of the researchers’ prototypes was made with about 1,000 300-micron-sized holes, and it can hold about 30 grams, which is more than 70 paper clips. They found that as they scaled down the holes and packed more of them onto the device, the adhesion got stronger. They estimate, then, that a one-square-inch device with millions of 1-micron-sized holes could hold more than 15 pounds. “We think, eventually, we can get to sizes that can hold a human,” Steen said.
Reversing the electric field pulls the water back through the pores and breaks the tiny “bridges” created between the device and the other surface by the individual droplets. This turns off the adhesion.
One of the biggest challenges in making the device work was keeping the droplets from coalescing, as water droplets tend to do when they get close together, Steen said. To solve this, the researchers designed their pump to resist water flow while it’s turned off.
To go from grabber to pusher requires covering the droplets with a thin membrane – thin enough to be controlled by the pump, but thick enough to eliminate wetting.
“Imagine these droplets covered with a thin little rubber or elastic membrane, like a water balloon,” Steen said. “It can now push. It can’t grab the other side because it’s not exposed anymore. So now you’re using this guy as a pusher, rather than a grabber. The beetle taught us the beauty of this parallel action.
“Think about making a credit card-sized device that you can put in a rock fissure or a door, and break it open with very little voltage, or a firefighter who wants to get into a blocked room,” Steen added.