Van der Waals forces, named after a Dutch physicist of the late 1800s, are weak electrodynamic forces that operate over very small distances but bond to nearly any material.
Geckos have millions of setaemicroscopic hairs on the bottom of their feet. These tiny setae are only as long as two diameters of a human hair. That's 100 millionth of a meter long. Each seta ends with 1,000 even tinier pads at the tip. These tips, called spatulae, are only 200 billionths of a meter widebelow the wavelength of visible light.
"Intermolecular forces come into play because the gecko foot hairs split and allow a billion spatulae to increase surface density and come into close contact with the surface. This creates a strong adhesive force," said Autumn.
A single seta can lift the weight of an ant. A million setae, which could easily fit onto the area of a dime, could lift a 45-pound child. If a gecko used all of its setae at the same time, it could support 280 pounds.
"Our previous research suggested that van der Waals force could explain gecko adhesion. But we couldn't rule out water adsorption or some other types of water interaction. With our new data, we can finally disprove a 30-year-old theory based on the adhesion of water molecules," Autumn said.
The team's previous research ruled out two other possible forms of adhesion: suction and chemical bonding.
Geometry versus Chemistry
"The van der Waals theory predicts we can enhance adhesionjust as nature hassimply by subdividing a surface into small protrusions to increase surface density," Autumn explained. "It also suggests that a possible design principle underlies the repeated, convergent evolution of dry adhesive microstructures in geckos, anoles, skinks, and insects. Basically, Mother Nature is packing a whole bunch of tiny things into a given area."
If van der Waals adhesion determines setal force, then geometry and not the material make-up should dictate the design of setae, the team predicted.
Jacob Israelachvili at the University of California at Santa Barbara applied a mathematical modelthe Johnson-Kendall-Roberts theory of adhesionto predict the size and shape of the setae.
Ronald Fearing at the University of California at Berkeley took the empirical results and nanofabricated synthetic foot-hair tips from two different materials.
"We confirmed it's geometry, not surface chemistry, that enables a gecko to support its entire body with a single toe," Autumn said.
"This means we don't need to mimic biology precisely," he explained. "We can apply the underlying principles and create a similar adhesive by breaking a surface into small bumps. These preliminary physical models provide proof that humans can fabricate synthetic gecko adhesive," he said.
This story aired on our U.S. cable television program National Geographic Today
"The artificial foot-hair tip model opens the door to manufacturing dry, self-cleaning adhesive that works under water and in a vacuum," according to Autumn, who foresees countless applications for synthetic gecko adhesivefrom vacuum areas of clean rooms to outer space.