Science is most fun when it tickles your imagination. This is particularly true when you see something you know just can’t be true. I once saw Fred Astair dance across a ceiling, and the little scientist in my head stopped humming along to the movie’s music and started shouting “How did he do that!” The answer soon popped into my mind: “Its a movie, stupid, just a clever special effect, probably done with mirrors,” and I went back to enjoying Astair’s dancing.
I got tickled a lot more forcefully some years later, on my tenth wedding anniversary. Barbara and I were someplace tropical, and I was lying on a bed while Barbara was outside doing something strenuous, when a little lizard walked up the wall beside me and across the ceiling, stopping right over my head and looking down at me. I swear I could see an amused Fred Astair peering down at me through its eyes.
This was no special effect, no trick with mirrors. I was seeing it with my own eyes, real as day, right there above me. The lizard, a blue gecko about the size of a toothbrush, stood upside down on the ceiling and seemed to laughed at me for several minutes before trotting over to the far wall and down.
How did my gecko perform this gripping feat? Investigators have puzzled over the adhesive properties of geckos for decades. What force prevented gravity from dropping the gecko on my nose?
The most reasonable hypothesis seemed suction — salamanders’ feet form suction cups that let them climb walls, so maybe geckos do too. The way to test this is to see if the feet adhere in a vacuum, with no air to create suction. Salamander feet don’t, but gecko feet do. Its not suction.
How about friction? Cockroaches climb using tiny hooks that grapple onto irregularities in the surface, much as rock climbers use crampons. Geckos, however, happily run up walls of smooth polished glass that no cockroach can climb. Its not friction.
Electrostatic attraction? Cloths in a dryer stick together because of electrical charges created by their rubbing together. You can stop this by adding a “static remover” like cling-free that is itself heavily ionized. But a gecko’s feet still adhere in ionized air. Its not electrostatic attraction.
Could it be glue?. Many insects use adhesive secretions from glands in their feet to aid climbing. But there are no gland cells in the feet of a gecko, no secreted chemical. Its not glue.
There was one tantalizing clue, however, the kind that experimenters love. Gecko feet seem to get stickier on surfaces with highly-ordered molecules. This suggests that geckos are tapping directly into the molecular structure of the surfaces they walk on! Tracking down this clue, Robert Full of the University of California, Berkeley, and his research team took a closer look at gecko feet. Geckos have rows of tiny hairs on the bottoms of their feet, like the bristles of some trendy toothbrush. There are about half a million of these hairs on each foot, pointed towards the heel
When you look at these hairs under a microscope, the end of each hair is divided into between 400 and 1,000 fine projections, the projections sticking out from the tip like tiny stiff brushes.
When a gecko takes a step, it drives the sole of its foot into the surface and pushes it backwards. This shoves the forest of tips directly against the surface. The atoms of each gecko tip become closely engaged with the atoms of the surface, and THAT is the force that defies gravity. When two atoms approach each other very closely — closer than the diameter of an atom — a subtle nuclear attraction called Van der Waals forces comes into play. These forces are individually very weak, but when lots of atoms add their little bits, the sum can add up to quite a lot.
Full and his team used microelectrical mechanical sensors (originally designed to be used with atomic force microscopes) to measure the force exerted by a single hair removed from a gecko’s foot. It was 200 micronewtons, a tiny force but stupendous for a single hair. Enough to hold up an ant. A million hairs could support a small child. My little gecko, ceiling-walking with two million of them, could have carried an 80 pound backpack — talk about being over-engineered.
If they stick that good, how do geckos ever come unstuck? For a gecko’s foot to stick, each hair projection must butt up squarely against the surface, so the hair’s individual atoms can come into play. Tipped past a critical angle — 30 degrees — the attractive forces between hair and surface atoms weaken to nothing. The trick is to tip the foot hairs until the projections let go. Geckos release their feet by curling up each toe and peeling it off, sort of like undoing velcro.
Now I can laugh with my little gecko friend, should I see it again, for I know its secret.
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