When we observe a gecko, a reptile from the lizard family, crawling up and down walls at a speed of several meters per second, it seems to have some superpowers that defy the laws of physics. It effortlessly climbs all types of surfaces and can stay on them with just one finger. How does it manage to do that? This marvel of evolution was already noticed by the Greek philosopher Aristotle in the 4th century BC, and it has not left today’s researchers indifferent. Its ability to stick to surfaces cannot, of course, contradict the laws of physics, which were yet to be discovered. Until 2002, experts attributed the gecko’s adhesion ability to the capillary forces between the substrate and the hair on its feet, but research has shown that this force does not act on all surfaces the gecko can move on. Therefore, this theory was rejected. Further research, using an electron microscope, has proven that the so-called Van der Waals forces are responsible for the phenomenon of dry adhesion. The forces that act between the gecko’s hairs and the surface have been studied. It has been shown that the gecko uses only 3% of its hairs for adhesion, which is understandable. Namely, not all of its hairs can participate at the same time because they cannot all be directed equally, and on a rough surface, not all hairs can be in even contact with the surface. The adhesive forces of these hairs are strong enough that even such a small percentage allows the gecko to firmly adhere. The remaining hairs come in handy, especially during strong tropical winds, in defense against predators, and in possible falls, where the gecko can grab onto leaves or branches. The adhesion that the gecko establishes with the surface is direct and works on any surface (even in a vacuum or underwater), without the gecko releasing any sticky substances.
Mechanism of adhesion on the feet
The gecko’s secret lies in the unusual natural mechanism that this species has developed. During evolution, their feet have shaped in such a way that Geckos have millions of tiny hair-like structures that allow them to move on almost any surface. This is one of the most effective and adaptable mechanisms of adhesion, enabling movement on rough and smooth surfaces in all directions. Each hair on the foot of the most studied gecko is approximately 110 µm long and 4.2 µm wide, with a tip that is about 200 nm long (the same width), shorter than the wavelength of visible light. The forces that enable this adhesion mechanism are called Van der Waals forces, which allow the gecko to cling to a wall with just one finger. However, these forces are actually very weak. How is it that they can hold the gecko on a surface, but a human cannot stick to a wall with their palm? The key role is played by the hairs themselves, which are so dense and thin that their total contact area is extraordinarily large, while our palm, pressed against the wall, only touches the wall at a few points.
Van der Waals Forces
Van der Waals forces are weak attractive forces between molecules. ich the Van der Waals forces that arise due to the uneven distribution and movement of electrons in atoms and molecules. There are three types of Van der Waals bonds: orientational, inductive, and dispersive. The overall Van der Waals bond is the result of the influence of all three types of bonds. To better explain these laws, let’s simplify and imagine just one strand of hair pressed against a wall. Of course, there are more atoms at the end of the hair, but let’s take only one atom as an example. It becomes polarized (for example, the positive charge gets closer to the wall) and therefore attracts the electron (negative charge) of the nearest atom on the wall. In this way, an attractive force is created between the positive and negative charges. Since there are thousands of atoms at the end of the hair, their continuous movement establishes a continuous attractive force. According to calculations, the force acting between each individual hair and the wall is approximately 20 µN (measured at 40 µN [Prof. Kellar]), and since all four legs have a total of about 6.5 million hairs, this means that the total attractive force is The adhesive force is 130 N (which could hold a mass of 13 kg). Therefore, the maximum mass a gecko could hold on the wall is 43 times greater than its own mass, meaning it would be able to hold itself up with just one finger.
How to release this strong grip?
The gecko effortlessly detaches from the wall in a tiny 15 milliseconds. When it lifts its foot, the distance between the hair and the wall increases, causing the weak Van der Waals forces between molecules to loosen. However, detaching all the hairs at once is too difficult because the sum of all the forces is significant. Since individual hairs have little significance in this overall number, the task is simplified by gradually detaching the hairs. That is why the gecko utilizes two mechanisms that weaken the adhesive forces. One of the mechanisms is similar to peeling off adhesive tape. Namely, the gecko moves by bending its feet, gradually lifting them from the tips of its toes towards the center of its feet, thus breaking the forces. Another mechanism that scientists have discovered is that with increasing angle. temelji i robotike. And sports as well.
Organized by: Jerica Jerič