A gecko sticks without glue

A gecko’s toe is covered in hundreds of thousands of hairs, split at the tips into still smaller pads. The force holding it to the wall is intermolecular attraction, and the toe lets go by changing its angle.

Each seta on a gecko’s toe splits into hundreds of spatulae, flat ends a few hundred nanometres across. Pressed to a surface, those ends sit close enough for van der Waals forces — the ordinary attraction between atoms — to add up. Kellar Autumn and colleagues measured a single seta and found it could lift tens of micronewtons, and that the force did not require a polar surface, which argued against water-based capillary glue as the main agent. A foot is the sum of hundreds of thousands of those hairs. The sum holds the animal. Sometimes it holds more than the animal.

Letting go is the clever half. Pull the hair past a critical angle and the pads peel, the contact area collapses, and the force disappears. The gecko walks by attaching and peeling faster than the eye follows, on glass, on stone, and upside down. Dirt and very rough surfaces and a few engineered coatings reduce the force. The principle does not. It is geometry, aimed at a force that is usually too small to notice.