Transcription
Translator: DAHOU Mohamed
Reviewer: khalid marbou
It is midnight, and everything is still, except for the soft footsteps of a gecko hunting a spider. The gecko appears to defy gravity as it climbs vertical surfaces and walks upside down without claws, adhesive secretions, or strong spider webs. Instead, it utilizes a simple principle: the attraction of positive and negative charges. Gravity brings elements together, like salt, composed of positively charged sodium ions that stick to negatively charged chloride ions. But the gecko's feet have no charges, nor do the surfaces it walks on. So, what makes it stick in that manner?
The answer lies in a clever combination of intermolecular forces and structural engineering. All elements in the periodic table have a different attraction for electrons. Elements like oxygen and fluorine strongly need electrons, while elements like hydrogen and lithium do not attract them as intensely. This relative attraction of an atom for electrons is called electronegativity. Electrons are constantly moving and easily transfer to where they are needed most.
Therefore, when there are electrons with different electronegativities in a single molecule, the electron cloud is attracted towards the more electronegative atom. This creates a small spot in the electron cloud, allowing the atom's nucleus to radiate a positive charge, in addition to a negative charge from electrons elsewhere. This means the molecule itself is not charged, but it possesses patches of positive and negative charges. These charged patches attract neighboring molecules. They align so that positive patches meet negative patches on the other side.
There is no need for a strongly electronegative atom to create these attractive forces. Electrons are in constant motion and sometimes temporarily accumulate in one spot. This fleeting charge is enough to draw molecules to each other. These interactions between uncharged molecules are called Van der Waals forces. They are not as strong as interactions between charged particles, but if you have enough of them, they can add up significantly.
This is the gecko's secret. The soles of a gecko's feet are made of flexible edges. They are covered with fine hairs, thinner than human hair, called setae. Each of these setae is covered with even finer hairs called spatulae. These spatula-shaped hairs are perfect for the gecko's needs: sticking when necessary. When the gecko spreads its flexible toes on the ceiling, the spatulae adhere at an ideal angle to create Van der Waals forces. The spatulae flatten to create many surface areas for their positive and negative charge patches to find suitable patches on the ceiling.
Each spatula contributes a small amount of Van der Waals force, but a gecko has about two billion spatulae, which collectively create enough force to support its weight. In fact, a gecko can hang by a single toe. Yet, it can also cancel this remarkable adhesion by slightly changing the angle of adhesion. This allows it to move its toe to strike at its prey or escape a predator.
This strategy, which uses a forest of hairs to maximize Van der Waals forces between ordinary molecules, has inspired the creation of equipment designed to mimic the gecko's amazing ability to adhere. Although artificial versions are not yet as strong as a gecko's adhesion, they are good enough to allow an adult to climb an 8-meter-high glass wall. In fact, the gecko's prey also uses Van der Waals forces to stick to the ceiling. Therefore, the gecko cancels the adhesion of its toes to launch itself for the hunt.