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  • It's midnight and all is still, except for the soft skittering of a gecko hunting a spider.

  • Geckos seem to defy gravity, scaling vertical surfaces and walking upside down without claws, adhesive glues or super-powered spiderwebs.

  • Instead, they take advantage of a simple principle: that positive and negative charges attract.

  • That attraction binds together compounds, like table salt, which is made of positively charged sodium ions stuck to negatively charged chloride ions.

  • But a gecko's feet aren't charged and neither are the surfaces they're walking on.

  • So, what makes them stick?

  • The answer lies in a clever combination of intermolecular forces and structural engineering.

  • All the elements in the periodic table have a different affinity for electrons.

  • Elements like oxygen and fluorine really, really want electrons while elements like hydrogen and lithium don't attract them as strongly.

  • An atom's relative greed for electrons is called its electronegativity.

  • Electrons are moving around all the time and can easily relocate to wherever they're wanted most.

  • So when there are atoms with different electronegativities in the same molecule, the molecules cloud of electrons gets pulled towards the more electronegative atom.

  • That creates a thin spot in the electron cloud where positive charge from the atomic nuclei shines through, as well as a negatively charged lump of electrons somewhere else.

  • So the molecule itself isn't charged, but it does have positively and negatively charged patches.

  • These patchy charges can attract neighboring molecules to each other.

  • They'll line up so that the positive spots on one are next to the negative spots on the other.

  • There doesn't even have to be a strongly electronegative atom to create these attractive forces.

  • Electrons are always on the move, and sometimes they pile up temporarily in one spot.

  • That flicker of charge is enough to attract molecules to each other.

  • Such interactions between uncharged molecules are called van der Waals forces.

  • They're not as strong as the interactions between charged particles, but if you have enough of them, they can really add up.

  • That's the gecko's secret.

  • Gecko toes are padded with flexible ridges.

  • Those ridges are covered in tiny hair-like structures, much thinner than human hair, called setae.

  • And each of the setae is covered in even tinier bristles called spatulae.

  • Their tiny spatula-like shape is perfect for what the gecko needs them to do: stick and release on command.

  • When the gecko unfurls its flexible toes onto the ceiling, the spatulae hit at the perfect angle for the van der Waals force to engage.

  • The spatulae flatten, creating lots of surface area for their positively and negatively charged patches to find complimentary patches on the ceiling.

  • Each spatula only contributes a minuscule amount of that van der Waals stickiness.

  • But a gecko has about two billion of them, creating enough combined force to support its weight.

  • In fact, the whole gecko could dangle from a single one of its toes.

  • That super stickiness can be broken, though, by changing the angle just a little bit.

  • So, the gecko can peel its foot back off, scurrying towards a meal or away from a predator.

  • This strategy, using a forest of specially shaped bristles to maximize the van der Waals forces between ordinary molecules has inspired man-made materials designed to imitate the gecko's amazing adhesive ability.

  • Artificial versions aren't as strong as gecko toes quite yet, but they're good enough to allow a full-grown man to climb 25 feet up a glass wall.

  • In fact, our gecko's prey is also using van der Waals forces to stick to the ceiling.

  • So, the gecko peels up its toes and the chase is back on.

It's midnight and all is still, except for the soft skittering of a gecko hunting a spider.

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B2 US TED-Ed gecko charged negatively van attract

【TED-Ed】How do geckos defy gravity? - Eleanor Nelsen

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    稲葉白兎 posted on 2019/03/03
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