Decades-old explanation for the lizards’ supersticky feet is wrong, researchers say
Clinging upside down to polished surfaces is simple for geckos, but scientists’ grasp of the underlying forces behind this phenomenal adherence just became murkier. Researchers report that the adhesiveness of gecko feet is aided by static electricity, contrary to long-held beliefs.
The misconception over gecko adhesion dates back to 1934, says Yale University chemical engineer and study coauthor Hadi Izadi. A German scientist named W.D. Dellit wondered whether gecko adhesion was explainable by electrostatic forces, the differences in electric charge that build up between any two surfaces. Dellit used X-rays on the air surrounding the reptile’s toes as they stuck to a metal wall. The X-rays ionized the air, neutralizing any charge on the wall’s surface, Izadi explains.
Anything attached to the wall via electrostatic forces should have fallen, yet the lizards remained. So researchers ruled out electrostatic forces and moved on to other ideas. Over a decade ago, a pair of studies asserted that geckos owe their stickiness to van der Waals forces (SN: 7/15/00, p. 47). These are the weak interactions between molecules that naturally exist between any touching surfaces, such as a smooth windowpane against a gecko’s setae — little toe hairs that are responsible for the lizard’s freakish grip (SN: 8/31/02, p. 133).
But scientists may have been too hasty in ruling out electrostatic forces, Izadi says. The gap between a smooth surface and a gecko’s setae is too small for destabilizing air ions to pass through, which wasn’t known in Dellit’s era. So his X-ray experiment probably didn’t counteract electrostatic forces that may have helped the geckos stick.
As a graduate student at Canada’s University of Waterloo, Izadi compared how the footpads of tokay geckos (Gekko gecko) stuck to two polymer surfaces. One surface consisted of Teflon AF, a material related to the nonstick cookware coating, while the other was made from a silicone rubber dubbed polydimethylsiloxane.
Because of their chemical makeup, both substances should have had similar degrees of van der Waals interactions with the geckos’ setae, and therefore the animals should have clung similarly to both surfaces. But when the researchers gently pulled a lizard’s foot away, the footpads stuck to Teflon AF with twice as much strength as they did to polydimethylsiloxane, Izadi and his colleagues report July 9 in the Journal of the Royal Society Interface. Because the adhesions were very different, the team concluded that van der Waals forces don’t fully explain how geckos stick to walls.
Next, the researchers examined whether electrostatic forces could account for this discrepancy by measuring the charge between each polymer surface and the gecko’s feet. The geckos’ toe pads and the polymer surfaces were electrically neutral before touching. But when they came in contact, electrons jumped from the gecko foot to the polymers, leaving the foot positively charged and the polymers negatively charged. The team also ruled out water-governed capillary forces, leading to the conclusion: “Electrostatic interactions are the dominant forces, and they are not something that scientists can ignore,” Izadi says.
But some scientists remain skeptical. While electrostatic forces may contribute to gecko adhesion on some surfaces, the claim that they are dominant is probably incorrect, says biologist Kellar Autumn of Lewis and Clark College in Portland. Autumn led the team that first declared van der Waals forces as the primary source of gecko stickiness. He points out that geckos can adhere to surfaces where electric charges don’t accumulate, such as bare steel and underwater. “These observations demonstrate that contact electrification is not required for strong adhesion,” he says.
Electrical engineer Ronald Fearing of the University of California, Berkeley, who studies geckos and makes gecko-inspired adhesives, agrees that van der Waals forces must still play a role. “Of course,” he says, “you may just find situations where the electrostatic becomes larger than van der Waals forces.”
H. Izadi, K.M.E. Stewart and A. Penlidis. Role of contact electrification and electrostatic interactions in gecko adhesion. Journal of the Royal Society Interface. Published online July 9, 2014. doi: 10.1098/rsif.2014.0371.