Grades 6–8 · Organisms
How can a gecko hold on without glue?
A gecko can cling to a smooth wall and then lift a foot for its next step. The gripping surface is not a coating of wet glue. On the toe pads of many geckos, a hierarchy of small structures makes remarkably close contact possible.
In the diagramTiny contacts make a powerful grip▸ Toe padsBranched hairsSurfaceClose-contact attractionToe pads carry many small structures.
Microscopic hairs called setae branch into even smaller flattened tips, often called spatulae. A large rigid foot would touch an uneven surface at relatively few places. Numerous flexible tips can conform much more closely, creating many small regions of contact instead.
At extremely short distances, molecules in a tip and molecules in the surface attract through van der Waals interactions. Each contact contributes a small force. Many suitably engaged contacts together can support substantial loads. The importance of close contact explains why the shape and arrangement of the toe structures matter, not just the material they are made from.
The system also allows release. The gecko changes the direction and angle of its toes and peels contacts away rather than pulling every tip off at once. The illustration magnifies this contact mechanism; a few drawn branches do not measure a whole animal’s grip on every wet, dirty or rough surface.
About this explanation
Starting knowledge: Understands contact area, forces and that microscopic structure can change a material’s behavior.
Sources
- Autumn and colleagues, PNAS — Evidence for van der Waals adhesion in gecko setae
- University of California, Berkeley — To climb like a gecko, robots need toes