How do geckos' feet adhere to smooth surfaces?

How do geckos' feet adhere to smooth surfaces?

Geckos can walk on walls or even ceilings. The 3D scene explains this ability.

Biology

Keywords

gecko, terrarium, Van der Waals, adhesion, secondary bonding, keratin scale, animal, vertebrates, reptile, insectivore, nectarivore, biology

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Scenes

Gold dust day gecko

  • Gold dust day gecko - This species of gecko, native to northern Madagascar and nearby islands, is one of the most colourful species of gecko. It was named after the yellow spots on its back. The animal is 10-13 cm long and weighs 40-80 g. Its body is yellow-green or bright green, while its legs are mostly blue. It lives mainly on banana plants and palm trees. It is active during the day and feeds on insects, smaller reptiles and fruits.

People have been fascinated since ancient times and tried to understand the geckos' ability to walk confidently on the smooth surfaces of walls or even ceilings. However, it was only at the beginning of the 21st century, that scientists could answer this question. The answer lies in the very special anatomy of the geckos' solepads. The strong adhesion between geckos' feet and smooth surfaces is a result of the attraction forces, known as van der Waals forces, among molecules.

Anatomy of the animal

  • foot - The solepads are covered with tiny setae which end in several billions of spatulas. These ensure strong adhesion through the van der Waals interaction, this is why geckos can walk even on the ceiling.
  • opening of the auditory canal
  • blue skin around the eyes
  • reddish dorsal pattern
  • Length: 10–13 cm
  • slightly flattened tail - Geckos can drop their tails if their life is in danger. This phenomenon is called autotomy.

Anatomy of the foot

  • setae - Their diameter is about 5 micrometres, while that of the thinnest human hair is about 19 micrometres. Each square millimetres of the solepads contains about 14,000 setae, the 4 legs contain about 6.5 million of them altogether.
  • seta - Their diameter is about 5 micrometres, while that of the thinnest human hair is about 19 micrometres. Each square millimetres of the solepads contains about 14000 setae, the 4 legs contain about 6.5 million of them altogether.
  • spatulae - Each seta ends in 100–1,000 of these, therefore there are several billions of spatulas on the soles. Weak van der Waals bonds form between the spatulas and the surface molecules, which ensure strong adhesion. In theory, they can hold more than one hundred kilos of weight. To detach the foot from the surface the angles of the setae have to be changed; this way a smaller force is enough to detach the foot, as the bonds are not broken simultaneously, but gradually.
  • spatula
  • van der Waals interaction - Weak bonds form between the surface molecules and the ends of the spatulas. The reason for this is the vibration of nuclei: temporary shifts of charge are formed in apolar molecules, thus the molecules are temporarily polarised. This causes the next apolar molecule to become polarised, and electric attraction is formed between the two molecules.

Attachment and detachment

  • spatula
  • van der Waals interaction - Weak bonds form between the surface molecules and the ends of the spatulas. The reason for this is the vibration of nuclei: temporary shifts of charge are formed in apolar molecules, thus the molecules are temporarily polarised. This causes the next apolar molecule to become polarised, and electric attraction is formed between the two molecules.
  • attachment
  • detachment

Animation

Geckos are reptiles often kept as pets and known for their ability to walk on walls or even on the ceiling. This special ability is due to the anatomy of their toes.

The solepads are covered with setae, the diameter of which is about 5 micrometres, thus they are much thinner than human hair, which is about 19 micrometres in diameter. The 4 feet contain about 6.5 million setae altogether. Each seta ends in several hundreds or thousands of spatulae, thus the four feet contain several billions of them. When the gecko places its foot on a surface, weak van der Waals bonds form between the spatulae and the surfaces. Due to the large number of spatulae, this creates strong adhesion: in theory a gecko’s four feet could hold more than one hundred kilos of weight.

To detach the foot from the surface the angles of the setae have to be changed. A smaller force is sufficient to do this as not all the bonds are broken at the same time.

Van der Waals interaction

  • electron cloud
  • nucleus - The positively charged nucleus and negatively charged electron cloud of the atom vibrate, thereby creating temporary dipoles.
  • electrostatic interaction - When two particles get close and one is polarised, its positive pole repels the nucleus of the other particle and attracts its electron cloud. Thereby it induces polarisation in the other particle and electrostatic interaction occurs between them. This interaction is quite weak, the weakest of the secondary chemical bonds. However, due to the large surface area of the spatulas, it provides strong adhesion.

The adhesion of the spatulae on the surface is provided by van der Waals forces. The basis for this interaction is the oscillation of the positively charged nuclei and the negatively charged electron cloud in the atoms. When a particle of the gecko’s foot and a particle of the glass surface come in close proximity, one of them becomes polarised; its positive pole repels the nucleus of the other particle and attracts its electron cloud. Thereby it induces polarisation in the other particle and electrostatic interaction occurs between them. This interaction is quite weak, the weakest of the secondary chemical bonds. However, due to the large surface area of the spatulae it provides strong adhesion.

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