Van der Waals helps geckoes scale walls
Face it, it would be totally cool if we could clamber up surfaces as easily as geckoes do. We could scale skyscrapers, never fear when climbing ladders, and could completely eliminate that tacky dramatic moment in movies where the hero dangles precariously over the street a hundred storeys below. Of course, their sweaty fingers would never slip if they had some kind of adhesion mechanism—they could just climb right back up.
So how do geckoes manage it?
Well, unlike humans, geckoes have millions of microscopic hairs on the bottom of their feet, called setae. The tips of each of these setae are split into 100-1000 spatulae, which are so small that they’re narrower than the wavelength of visible light—less than 300 nano metres.
Clearly, some kind of intermolecular force between the gecko’s feet and a surface is responsible for adhesion, but it wasn’t until research in 2002 that we fully understood what was going in—for a while, scientists were throwing around theories like suction and chemical bonding.
Turns out, geckoes take advantage of the Van der Waals force.
Named after a nineteenth century Dutch physicist, Van der Waals forces are weak electrodynamic forces that act over tiny distances, yet bond almost any material. They’re created by fluctuations in charge distributions between molecules.
These weak forces can be strengthened as more and more of one surface touches the other—like, say if you had billions of spatulae coating your feet. These tiny hairs increase surface density, so on contact with the wall the gecko experiences a strong adhesive force
Essentially, this force means we can improve adhesion simply by increasing surface density, like subdividing a surface into countless small protrusions. It means that geometry—not chemistry—is the driving mechanism. A single setae can lift an ant; a million could lift a 20 kg child; and if geckoes used every setae simultaneously, they could support 130 kg.
These forces open up to a lot of applications in adhesives. Engineers at Berkeley and Stanford have developed biologically inspired synthetic adhesives that adhere like gecko pads, which have even been used on robotic climbers.