Science-- there's something for everyone

Tuesday, February 2, 2010

Could people walk up walls?

Paul Steen of Cornell University and his former postdoctoral associate Michael Vogel have invented a powerful reversible adhesion system. Their work will be published in the Proceedings of the National Academy of Sciences.

The device, inspired by the Palmetto Tortoise Beetle (which uses its 10,000 adhesive bristles to attach itself to leaves with a force equal to 60 times its body weight), consists of several layers.  On the bottom is a plate containing a liquid reservoir, the middle is a porous layer and the top is a flat plate patterned with minute holes. When an electric field is applied via a nine-volt battery, water is pumped from the bottom forcing droplets to squeeze through the tiny holes on top.  The surface tension of these droplets causes them to grip another surface the way two wet glass plates will stick to each other.  To turn the adhesion off, the electric field is simply reversed, pulling the water back through the pores and breaking the surface tension.

Paul Steen and Michael Vogel's surface tension-based adhesive device with a lego man payload (Credit: Michael Vogel)

The smaller and more numerous the holes, the more powerful the adhesion affect was.  In their tests, 1000 holes, each 300 microns in diameter, could successfully lift 30 grams.  Extrapolating, the scientists predict that if covered with millions of one micron diameter holes, a one inch square device could hold over 15 lbs. 

Unfortunately, it becomes increasing difficult to prevent the water droplets from coalescing as they get smaller and more numerous. The scientists are working to overcome this problem.

By my calculations, a one square inch plate can contain a few hundred million micron-sized holes, assuming almost no space between them.  That might be enough to hold a person’s bodyweight, leading to speculation that this device could allow people to walk up walls. Personally, I’m not sure I’m ready for the kind of acrobatics it would take to move around using battery operated adhesive plates.  On the other hand, I do look forward to sticking all my belongings onto wall plates until I need them.