Physics in the animal world: gecko's paw

Photo: Wikimedia

Geckos are inhabitants of tropical and subtropical regions of the Old and New Worlds. These lizards live both on continents and on islands, the range of their distribution is extensive. In geckos there is one feature - they are able to stay on almost any surface. Even one paw holds the body weight of the animal. The surface can be any - wood, rock, even polished glass.

Even the ancient Greeks paid attention to the ability of a gecko to hold tight to anything. Aristotle tried to understand the principle of fastening the paws of a lizard, interested in geckos and medieval scientists. They are studied in our time. There are several theories explaining the outstanding abilities of these lizards in "mountaineering".

Sucker on fingers. One of the first explanations that seemed quite logical. However, after studying the gecko's paw under a microscope, it turned out that there are no suckers on the fingers. Unfortunately, the myth of suckers lives to this day.
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Electrostatics . Another plausible explanation that was managed to disprove (although there is some evidence of this theory, we will talk about them below), creating conditions under which the charge on the gecko’s legs simply could not be. The animal is still firmly kept on a smooth surface.

The refutation was received in the 30s of the last century. German scientist Wolf-Dietrich Dellit (Wolf-Dietrich Dellit) sent a stream of ionized air towards the paws of the gecko, which was kept on a metal surface. Ionization, according to Delita, should have neutralized or significantly reduced the strength of the clutch paws with the surface, if the clutch mechanism had an electrical nature. This did not happen, so it was concluded that geckos were using something else.

Canadian scientist Alexander Penlidis believes that this experiment was incorrect. The fact is that the contact between the gecko's paws and the surface is extremely small, as a result of which ionized molecules simply cannot penetrate between the ultra-small structures of the paws and the surface and neutralize the interaction.

The clutch paws gecko with surface irregularities . This explanation is also not appropriate, since geckos can move on a vertical surface of polished glass. Moreover, they can move along the ceiling of the same material.


Photo: wikipedia

With the advent of the electron microscope, the gecko's paw was studied in detail. As it turned out, it is covered with extremely thin bristles, whose length is up to hundreds of micrometers. The concentration of bristles per unit surface area of ​​the paw is very high: more than 14,000 hairs per 1 mm ² . Each seta, in turn, is not a monolithic formation, but is divided at the end into 400-1000 even smaller fibers. The thickness of such fibers is 0.2 microns. At 1 cm ^{2 of} contact with the surface there are about 2 billion fibers, each of which expands towards the end.


but. Gecko foot b. Gecko's "little pad" under a microscope c. One of the setae of the gecko's paws. It is, under a large magnification. The maximum increase in the bristles (photo: somuchnews )

American scientists have found that the force of adhesion of the paw of the gecko currents is 10 Newton per 1 cm ² . Such adhesion is possible only for smooth surfaces, where almost all the fibers on the legs of the animal are involved. If we are talking about surfaces that are often found in the habitats of geckos - rocks, trees, only a part of the fibers on the paws are involved (due to the large number of irregularities on these surfaces), but this is enough to keep the animal in place.

As it turned out, the microscopic hairs on the legs of the gecko interlock with the supporting surface by means of van der Waals forces. Van der Waals forces are forces of intermolecular (and interatomic) interactions with energies of 10–20 kJ / mol. The basis of the van der Waals forces are the Coulomb forces of interaction between electrons and nuclei of one molecule and the nuclei and electrons of another. At a certain distance between the molecules, the forces of attraction and repulsion balance each other, and a stable system is formed. Such a system is the gecko's paw with the surface with which it adjoins.

The complex structure of the paw provides another of its properties - hydrophobicity. The paw repels water and dirt, thanks to which the gecko can move well on wet surfaces.

The gecko easily detaches the paw from the surface on which it is attached. For this, a special mechanism is used. The fact is that a bristle attached to a material can easily loosen if the angle between the fiber and the surface is more than 30 °. When moving, changing the angle of contact between the paw and the surface, the gecko easily secures and unfastens the paws. Energy costs for this process are minimal.

Forces vand-der-vaals or something else?

Two years ago, Canadian scientist Alexander Penlidis decided to independently study the mechanism of sticking of gecko's paws to surfaces. As it turned out, the contact of the paw and the surface occurs the exchange of electric charges. As a result, a positive electrostatic charge is formed at the paw and a negative one at the surface.

Penlidis set up an experiment with two types of polymer surfaces - Teflon AF and polydimethylsiloxane. According to the findings made by scientists on the results of the study, the adhesion force correlated with the value of the electrostatic charge of the paw and the surface. And from this it follows that it is the electric charge that plays the main role in the clutch of the paw with the surfaces.



The study is interesting, but it does not answer the important question - how does a gecko stick on very uneven surfaces, where it is much more difficult to ensure adhesion using an electric charge than on a flat surface. Perhaps the gecko's feet have a double clutch mechanism — both van der Waals forces and an electric charge.

Water effect

In the overwhelming majority of cases, scientists conducted experiments with geckos in a dry environment. Scientists from the University of Akron decided to check how well the lizard can move on wetted surfaces. As it turned out, if you spray water on a glass plate, the animal keeps on such a surface much worse than on the same plate without water droplets.



However, the gecko manages to stay on the wet surface. But if the plate is immersed to a shallow depth in water, and the gecko is again placed on the plate, the lizard cannot hold onto the surface in such conditions. If the gecko's paws are immersed in water for an hour and a half, and then put it on the glass, it slides off, unable to consolidate.

According to Alyssa Stark from the University of Akron, this is because the water interferes with the forces of van der Waals interaction, and the gecko's legs cannot adhere to the surface.

Not only paws

The whole body of the gecko participates in the mechanism of fixing the paws on the surface, say scientists from the University of Massachusetts at Amherst. The body of a reptile, according to Alfred Crosby (Alfred Crosby), plays the role of a spring that presses the paws to the surface. And the greater the weight of the gecko, the stronger this spring . Thanks to this mechanism, large species of geckos, and not just their small relatives, are excellently kept in any surface.

Despite the fact that Alexander Penlidis was able to prove the effect of electric charge on the adhesion of the gecko's paws, most experts still support the point of view about the mechanism of adhesion based on van der Waals forces. Now scientists are trying to explain another interesting problem - the origin of this mechanism in the process of evolution.

"Gecko tape"

Since the mechanism of the gecko's legs as a whole has become clear, people are trying to reproduce it artificially. In particular, DARPA created climbing equipment that allows a person with a mass of 122 kg (body weight + payload) to climb a glass sheer wall to a height of 7.6 m. The Stanford engineer created a robot that can climb on almost sheer smooth surfaces. Robot manipulators are also modeled on gecko's paws. And experts from the University of Pennsylvania have developed a new type of high-precision gripper, which can be used in production for working with small parts. The development of superglue tape, which can withstand many cycles of use and the surface of which is not contaminated with prolonged use, is also being carried out. NASA has developed a special mount that can be used both under the conditions of the Earth and in conditions of weightlessness in space. It allows you to attach loads to the surfaces with the help of a special "Velcro", created in the image and likeness of the surface of the gecko's paw.