The array, made up of polypropylene fibers packed together at 42 million per square centimeter, can hold a quarter from falling down a slide held at an 80-degree angle. The fibers, which measure 20 microns long and 0.6 micron in diameter, hold the quarter in place through friction. The fiber array is not sticky or goopy.
The research, led by graduate student Carmel Majidi, is part of the ongoing study into geckos, those tropical lizards that can walk up walls and across ceilings. Gecko feet are covered with millions of tiny hairs. When these hairs brush against a surface, the molecules at the tip of the hairs bond with the surface through van der Waals forces, a bond that forms between two molecules when they are close to each other. Although the bond between each hair and the surface is weak, it becomes strong when multiplied over millions of hairs.
At other universities, researchers are tinkering with van der Waals forces to ror create carbon nanotube memory chips.
Through their feet, geckos get both friction, which keeps them from sliding down, and adhesion, which makes it difficult to pull them off a surface. Scientists hope to better understand the forces behind gecko feet and replicate the principles in products.
The microfiber array developed at U.C. Berkeley doesn't exhibit adhesion. Thus, a suit of the material would not let you do Spider-Man stunts, but it would probably hold you up much better than a Velcro sweat suit. Nonetheless, as a high-friction surface, microfiber arrays could be incorporated into athletic shoes and tires.
"With rubber, you control friction and adhesive properties by changing its chemical formulations," U.C. Berkeley electrical engineering professor Ronald Fearing said in a statement. "For the microfiber array, we just change its geometry and mechanical properties. Thicker, fatter fibers, for instance, reduce the amount of friction created."