Begin p Seta pulled on sensor Time(s) The sliding step experiment. The adhesive force of a single seta was measured. An initial push perpendicularly put the seta in contact with the sensor. Then, with parallel pulling, Closeup look at a gecko's foot. The setae on a gecko's foot are continued to increase over time to a value of 60 microNewtons arranged in rows, and point backwards, away from the toenail. (after this, the seta began to slide and pulled off the sensor). In a Each seta branches into several hundred spatulae (inset photo) large number of similar experiments, adhesion forces typically ach 200 microNewtons Two hundred micro Newtons is a tiny force, but stupen dous for Igh to ho The Experiment up an ant. a million hairs could support a small child. a little ecko, ceiling walking with 2 million of them(see photos Once this team had isolated a seta and placed it in Kenny's above), could theoretically carry a 90-pound backpack--talk device, "We had a real nasty surprise, "says Autumn. Fc about being over-engineered wo months, pushing individual seta against a surface, they If a gecko's feet stick that good, how do geckos ever couldnt get the isolated hair to stick at all become unstuck? The research team experimented with This forced the research team to stand back and think unattaching individual seta; they used yet another micro- bit. Finally it hit them. Geckos don,t walk by pushing their instrument, this one designed by engineer Ronald Fearin feet down, like we do. Instead, when a gecko takes a step, it also from U C. Berkeley, to twist the hair in various way pushes the palm of the foot into the surface, then uncurls They found that tipped past a critical angle, 30 degrees, its toes, sliding them backwards onto the surface. this he attractive forces between hair and surface atoms shoves the forest of tips sideways against the surface. weaken to nothing. The trick is to tip a foot hair until its Going back to their instruments, they repeated their ex- projections let go. Geckos release their feet by curling up riment,but this time they oriented the seta to approach each toe and peeling it off, just the way we remove ta the surface from the side rather than head-on. This had the What is the source of the powerful adhesion of gecko feet? effect of bringing the many spatulae on the tip of the seta The experiments do not reveal exactly what the attractive into direct contact with the surface force is. but it seems almost certain to involve interactions at To measure these forces on the seta from the side, as well the atomic level. For a geckos foot to stick, the hundreds of as the perpendicular forces they had already been measur- spatulae at the tip of each seta must butt up squarely against ing, the researchers constructed a micro-electromechanical the surface, so the individual atoms of each spatula can come cantilever. The apparatus consisted of two piezoresistive into play. When two atoms approach each other very yers deposited on a silicon cantilever to detect force in closely--closer than the diameter of an atom-a subtle nu- both parallel and perpendicular angles clear attraction called Van der Waals forces comes into play These forces are individually very weak, but when lots of The results them add their little bits, the sum can add up to quite a lot. Might robots be devised with feet tipped with artificial With the seta oriented properly, the experiment yielded re- setae, able to walk up walls? Autumn and Full are working sults. Fantastic results. The attachment force measured by with a robotics company to find out. Sometimes science is the machine went up 600-fold from what the team had not only fun, but can lead to surprising advances en measuring before. A single seta produced not the microNewtons of force predicted by the whole-foot m To explore this experiment further surements, but up to an astonishing 200 micro Newtons go to the virtual lab at (see graph above)! Measuring many individual seta, adhe www.mhhe.com/raven6/vlabl.mhtml sive forces averaged 194+25 microNewtonsThe Experiment Once this team had isolated a seta and placed it in Kenny’s device, “We had a real nasty surprise,” says Autumn. For two months, pushing individual seta against a surface, they couldn’t get the isolated hair to stick at all! This forced the research team to stand back and think a bit. Finally it hit them. Geckos don’t walk by pushing their feet down, like we do. Instead, when a gecko takes a step, it pushes the palm of the foot into the surface, then uncurls its toes, sliding them backwards onto the surface. This shoves the forest of tips sideways against the surface. Going back to their instruments, they repeated their ex￾periment, but this time they oriented the seta to approach the surface from the side rather than head-on. This had the effect of bringing the many spatulae on the tip of the seta into direct contact with the surface. To measure these forces on the seta from the side, as well as the perpendicular forces they had already been measur￾ing, the researchers constructed a micro-electromechanical cantilever. The apparatus consisted of two piezoresistive layers deposited on a silicon cantilever to detect force in both parallel and perpendicular angles. The Results With the seta oriented properly, the experiment yielded re￾sults. Fantastic results. The attachment force measured by the machine went up 600-fold from what the team had been measuring before. A single seta produced not the 20 microNewtons of force predicted by the whole-foot mea￾surements, but up to an astonishing 200 microNewtons (see graph above)! Measuring many individual seta, adhe￾sive forces averaged 194+25 microNewtons. Two hundred microNewtons is a tiny force, but stupen￾dous for a single hair only 100 microns long. Enough to hold up an ant. A million hairs could support a small child. A little gecko, ceiling walking with 2 million of them (see photos above), could theoretically carry a 90-pound backpack—talk about being over-engineered. If a gecko’s feet stick that good, how do geckos ever become unstuck? The research team experimented with unattaching individual seta; they used yet another micro￾instrument, this one designed by engineer Ronald Fearing also from U.C. Berkeley, to twist the hair in various ways. They found that tipped past a critical angle, 30 degrees, the attractive forces between hair and surface atoms weaken to nothing. The trick is to tip a foot hair until its projections let go. Geckos release their feet by curling up each toe and peeling it off, just the way we remove tape. What is the source of the powerful adhesion of gecko feet? The experiments do not reveal exactly what the attractive force is, but it seems almost certain to involve interactions at the atomic level. For a gecko’s foot to stick, the hundreds of spatulae at the tip of each seta must butt up squarely against the surface, so the individual atoms of each spatula can come into play. When two atoms approach each other very closely—closer than the diameter of an atom—a subtle nu￾clear attraction called Van der Waals forces comes into play. These forces are individually very weak, but when lots of them add their little bits, the sum can add up to quite a lot. Might robots be devised with feet tipped with artificial setae, able to walk up walls? Autumn and Full are working with a robotics company to find out. Sometimes science is not only fun, but can lead to surprising advances. To explore this experiment further, go to the Virtual Lab at www.mhhe.com/raven6/vlab1.mhtml 1 2 Time (s) 345 20 0 -20 40 60 Force (µN) 80 0 Begin parallel pulling Seta pulled off sensor The sliding step experiment. The adhesive force of a single seta was measured. An initial push perpendicularly put the seta in contact with the sensor. Then, with parallel pulling, the force continued to increase over time to a value of 60 microNewtons (after this, the seta began to slide and pulled off the sensor). In a large number of similar experiments, adhesion forces typically approach 200 microNewtons. Closeup look at a gecko’s foot. The setae on a gecko’s foot are arranged in rows, and point backwards, away from the toenail. Each seta branches into several hundred spatulae (inset photo)