How Can a Gecko Cling to Smooth Walls and Glass?
The Molecular Force That Needs No Glue or Suction Cups
A gecko happily walks upside down across a window pane. Its feet have no sticky glue and no octopus-style suckers. So how does it keep clinging to smooth glass?
Rub a plastic sheet on your hair and it pulls your hair towards it. That "static electricity" is stronger than what we're talking about here, but it turns out that the molecules of every material attract one another very slightly.
Normally the pull is too weak to notice. But what if you gathered a countless number of these pulls together? That is exactly what happens on a gecko's foot.
Under each gecko toe are extremely fine hairs (setae), each one branching into even finer tips. A single toe is thought to carry hundreds of thousands of them.
When the tips come extremely close to a surface, a weak force that pulls molecules together (the van der Waals force) acts. Piled up countless times, it becomes strong enough to hold the gecko's body.
Let's look at these two ideas in turn: the countless fine hairs, and the weak forces that add up.
Why do the hairs branch so finely?
The van der Waals force, which pulls molecules together, is thought to act between all materials. But it is very weak, and it only shows itself clearly when molecules get extremely close. Under a microscope, ordinary surfaces are bumpy, so pressing two of them together brings only a tiny fraction of their area truly "close".
A gecko's foot is thought to solve this problem by branching its tips as finely as possible. The finer the tip, the more easily it slips into small bumps in the surface, so more tips can reach the distance at which molecules attract one another.
Suction cups stop working where there is little air, close to a vacuum. But experiments are reported to show that a gecko can cling to a wall even in near-vacuum conditions. Nor is it sticky like glue: it can stick and unstick again and again, and leaves no residue. These are clues that it clings by a different mechanism from either suction or glue.
It multiplies, by sheer numbers, an attraction that molecules already have.
Why can it let go so quickly?
With such strong adhesion, a gecko can still run along a wall, putting a foot down and lifting it many times a second. This is thought to be because the hairs grip strongly when pressed at one particular angle, but peel away with almost no force when the angle changes. It is much like sticky tape: pull it parallel to the surface and it barely comes off, but peel it up at a near-vertical angle and it lifts easily.
Try it yourself
- Stick a piece of clear tape onto a desk
- Pull the tape slowly at an angle close to parallel with the surface (it resists coming off)
- Pull the same tape at an angle close to perpendicular to the surface (it comes off easily)
- Confirm that, with the same adhesion, the force needed changes a great deal with the peeling angle
When a gecko lifts its foot, it is thought to use a similar trick: changing the angle so it can peel off without applying much force.
Summary
A gecko can cling to smooth walls not because it has special glue or suckers. It is thought to be because the countless hairs on its feet branch right down to the tips, a shape that makes the most of the van der Waals force between molecules. Each single pull is weak, but hundreds of thousands of them together add up to a force far beyond the gecko's body weight.
The secret of a gecko's foot is not strong glue, but a way of gathering weak forces in staggering numbers.
For another case where huge numbers make up for individual weakness, see our article on ants, which looks at muscle strength relative to body weight.
For those who want to know more: terms, numbers and links to textbooksFrom middle-school science to active research, each item is marked with its level
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school "Basic Physics"
- High school+High-school chemistry, or extension and sidebar material in textbooks
- UniversityUniversity-level specialist subjects (surface science, biomechanics) not taught in high school
- ResearchTopics researchers are still investigating, not yet taught as settled fact even at university
Middle schoolTerms: words used about gecko adhesion
- Seta (plural setae; Japanese: gōmō): An extremely fine hair-like structure growing under a gecko's toe.
- Spatula: An extremely fine, spatula-shaped structure formed where a seta's tip branches further.
- Van der Waals force: A weak attractive force acting between molecules.
High schoolChecking with a formula: is the foot's adhesion many times body weight?
From the adhesive force of one hair and the number of hairs on the whole foot, we estimate the theoretical total adhesion and compare it with body weight.
Theoretical total adhesion = adhesion per hair × number of hairs
| Adhesion per hair | About 200 micronewtons (a value often cited from experiments) |
| Number of hairs (all four feet) | About 500,000 (a commonly cited rough figure) |
| Total adhesion (micronewtons) | 200 × 500000 = 100000000 |
| Convert to newtons | 100000000 ÷ 1000000 = 100 |
| Result | Theoretically, about 100 newtons |
| Weight due to body mass (newtons) | 0.05 × 9.8 = 0.49 |
| Safety margin (times) | 100 ÷ 0.49 ≒ 204 |
| Result | Theoretically, enough to support about 200 times its body weight |
* In reality, not every hair on the foot is thought to deliver its maximum force at once, so this figure is close to a theoretical upper limit. Even so, it shows there is a very large margin over the force needed.
High school+Why does the force only work at close range?
The van der Waals force has the property that it weakens sharply as molecules move apart. Unless they get within a tiny, invisible distance (about one or two molecules' width), the force is barely felt. The extremely fine tips of the spatulae are thought to be a way of meeting this "extremely close" condition.
UniversityThe mechanics of adhesion and peeling in surface science and biomechanics
In surface science and biomechanics, researchers have studied in detail how the force needed changes with the angle at which setae touch and peel from a surface. Building on this, work is under way on artificial adhesive materials that imitate the gecko's foot (synthetic setae), for uses such as robots that climb walls.
ResearchWhat is still unclear
- The detailed mechanism by which a gecko's foot stays clean even with constant use (self-cleaning) is still being researched.
- How a gecko neurally controls the angle and force of its setae to put its feet down and lift them at high speed is also still being worked out.
- Technology to mass-produce artificial gecko-style adhesives with practical durability and cost is still under development in materials engineering.
Even one small gecko foot holds a rich, ongoing research topic where surface science and biomechanics meet.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: how living things are built | Basic terms: setae and spatulae |
| High school | Basic Physics: balance of forces | Comparing total adhesion with body weight |
| High school+ | Chemistry: intermolecular forces (advanced) | How force changes with distance |
| University | Surface science, biomechanics | Contact angle and peeling mechanics, artificial adhesives |
| Research | Materials engineering, biomimetics (ongoing research) | Understanding self-cleaning, control mechanisms, practical use |
- A classic study of gecko seta adhesion by Autumn, K. and colleagues.
- An explanation of the structure and adhesion mechanism of setae and spatulae, from biomechanics sources.
- An explanation of the development of artificial gecko-inspired adhesives, from materials engineering sources.
* The numbers of hairs and the adhesion per hair are commonly cited rough values. Actual values are said to vary with species, individual and measurement conditions.
* This article is a general-audience science explainer. The figures given are rough estimates to help you understand the mechanism. Actual values are said to vary with species, individual and measurement conditions.