|Gecko / Image credit: Chris Hunkeler|
It is almost impossible to read about van der Waals forces in a science textbook without also reading about geckos’ feet
. In general, van der Waals forces are the attractive and repulsive forces between two dipole molecules. Gecko feet are padded with microscopic fibers, which adhere to surfaces through van der Waals interactions. These fibers allow geckos to climb smooth surfaces like glass.
Scientists at the Department of Mechanical Engineering at the University of California Santa Barbara have successfully made a reusable dry adhesive inspired by the gecko’s feet. The results
were published on January 14, 2013 in the IOP journal Smart Materials and Structures
|Gecko toes / Image Credit: Matt Reinbold|
“We’ve been working a problem of this type...for about seven or eight years,” said Kimberly Turner, a professor of mechanical engineering and lead researcher on the paper. “I’m really interested in how nature has been able to achieve such incredible structures. The gecko is particularly interesting because...of its ability to stick on so many types of surfaces and to unstick so rapidly.”
The new adhesive, half-cylinder fibers made from a new silicon-based material called polydimethylsiloxane (PDMS), yields closer results than other similar adhesives developed in the past. Mimicking gecko feet, Turner’s new material easily sticks to smooth surfaces, adheres strongly when pushed forward and slides off when pulled backwards.
This directionality is due to the angle and shape of the fibers, fabricated using an angled photolithography technique created by Turner and her colleagues. Photolithography
etches a pattern on a thin photosensitive material using light—it’s “like taking a Polaroid picture,” said Turner. This process allows certain sections of the light sensitive material to be removed with a developer, much like photographic developer used to process film.
Using photolithography, the researchers carved out half-cylinder fibers of the silicon-based adhesive. When the fibers were pushed flat side down, the large surface area enabled the adhesive to stick firmly to the glass surface. When the fibers were pulled round side down, the limited surface area allowed the adhesive to slide off easily.
While Turner said that it would be ideal to get the high adhesion with very little application force, in reality, some initial pressure “is necessary to get the fibers to engage and bend over, providing the maximum van der Waals contact.”
The scientists next tested adhesion strength and durability and found that the adhesive retained 77 percent of its stickiness after 10,000 repeated test cycles. The material is the stickiest of gecko-inspired adhesives but falls short of gecko feet.
In the future, Turner hopes to tackle improvements to adhesion strength and to scale up the size of the adhesive strips so that the new material could be used “for mounting electronics on walls, to assembling parts of a robot, to mounting a painting.”
Jenni is a freshman at Swarthmore College, contributing to the APS Physic Buzz as a winter 2013 Swarthmore extern.