Six years ago, a team of researchers theorized that van der Waals forces helped geckos stick to walls and ceilings, since the millions of tiny hairs on their feet can form an intermolecular bond with any surface. This bond creates a force a thousand times stronger than what a gecko would need to adhere to a surface, the researchers said. Four years ago, members of the same team created an experimental verification for the theory of van der Waals forces and gecko foot adhesion by synthesizing sticking gecko hair tips.
What was not clear to the researchers, and what they could not recreate, was the gecko's ability to control this adhesion and attach and detach from a surface as many as 15 times per second, said Carmel Majidi, a UC Berkeley graduate student in electrical engineering and computer sciences and lead author of the paper.
Instead, the researchers created an array of synthetic polypropylene micro-fibers that can use friction to maintain contact with a smooth surface through even a light touch.
"We've taken a very stiff material that has no friction on its own, and by modifying the geometry alone, without altering any of its chemical properties, we were able to achieve this very high friction," said Majidi.
The researchers say that their array can create high friction and support loads on smooth surfaces -- the technology can hold up a quarter perched on a piece of glass angled at 80 degrees, but is not sticky to the touch. The micro-fibers are 20 microns long, have a diameter of 0.6 microns (roughly 100 times thinner than a human hair), and are packed in bunches of 42 million per square centimeter.
The more of these fibers that contact a surface, the more resistance and friction is generated, the researchers said. Each fiber produces 200 nanonewtons of shear resistance, while only 39 nanonewtons is required to bend the fibers. One nanonewton equals a force 1 billion times smaller than what Earth's gravity exerts on an average apple. To put the force in perspective, a keyboard stroke usually requires at least 700 million nanonewtons.
In the Aug. 19 issue of Physical Review Letters, the researchers noted that the micro-fiber array created friction -- the resistance of an object to being moved along a surface -- not adhesion, or the resistance of an object to being removed from a surface; the micro-fibers straighten and stiffen when pulled away from a surface.
This doesn't mean the micro-fiber arrays do not have practical applications, however. The researchers said that things like car tires and the soles of shoes would benefit from a product that has high friction but low adhesion. While rubber already provides these features to products, the researchers said a stiff polymer like the ones used for the micro-fiber array may be more resistant to wear and heat.
"With rubber, you control friction and adhesive properties by changing its chemical formulations," said Ronald Fearing, UC Berkeley professor of electrical engineering and computer sciences and principal investigator of the project, who has been on the team since the initial study six years ago. "For the micro-fiber array, we just change its geometry and mechanical properties. Thicker, fatter fibers, for instance, reduce the amount of friction created." He added that high-friction rubber also has the disadvantage of getting periodically sticky, unlike the polypropylene micro-fibers.
"Ultimately, what we'd like to have is something with high friction but can, with certain motions, also achieve adhesion," said Majidi.
The process of creating new technology by observing nature is called "biomimicry" or biomimetics.
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