Author: Andrew McAllister
Editors: Ana Vasquez, Molly Kozminsky, and Kevin Boehnke
One of the most frustrating parts of moving is dealing with furniture. Most pieces need to be taken apart to fit through doors or into your moving van. Even if you’re lucky enough to have buff friends to help, one lost or stripped screw is enough to make you question your choice to cart everything miles away.
If only things could be simpler. Instead of screws, why not a super strong, reusable, and easy-to-detach piece of tape to hold your furniture together? Sounds like a tall order, but scientists inspired by a gravity-defying lizard, the gecko, are trying to make it a reality.
How Geckos Harness van der Waals Forces to Climb Walls
Over the past century, scientists have mused about the mechanism that geckos use to cling to walls and other surfaces. Could it depend on friction, like the barbed legs of the cockroach? Secretions like a slug’s? Or suction, like a salamander’s feet?
Instead of using one of nature’s well-understood methods for sticking, geckos have evolved toes that take advantage of a phenomenon called the van der Waals force, a weak electromagnetic attraction between uncharged objects, such as your hand and a wall. That sounds strange, since our general rule of thumb about electromagnetic forces is that like charges repel, opposite charges attract, and objects with no charge aren’t subject to electromagnetic forces.
But the atoms that make up our body or the wall are made up of both positive nuclei and negative electrons. From far enough away, the charges of the nucleus and electrons cancel out and make the atom neutral, but up close, things aren’t so simple. Electrons are always in motion, and sometimes one side of an atom will become more electron-dense than the other side. The atom temporarily becomes what is known as a dipole, with one side bearing a slight positive charge and the opposite side a slight negative charge. This small charge imbalance can cause one electrostatically neutral atom to be slightly attracted to another for an instant (Figure 1).
The van der Waals attraction between two individual atoms is very weak. But if many atoms pair up, the total force can become strong enough to let a gecko stick to a wall!
Why We Can’t Be Like Geckos
But unlike a gecko, you can’t climb a wall or stand on the ceiling. What gives?
Think about pushing your fingertips against a wall. It feels like you are getting all of the atoms in your fingers close to all of the atoms in the wall. It certainly looks like you are touching everything underneath your fingertip. But in reality, even a surface that looks smooth, like glass, is rough and irregular on the atomic scale (Figure 2). This keeps most of the atoms in your fingertip too far away from the atoms in the wall for van der Waals forces to kick in.
Geckos get around this problem by having toes that are split up into many intricate structures called setae. Setae are like very thin human hairs with hundreds of split ends of varying lengths. Each one ends in many spatula-shaped structures that are small and flexible enough to fit between the tiny irregularities on the surface the gecko is trying to climb — allowing each one to get close enough to the surface to become attracted via the van der Waals force.
Visualize the difference between your hand and a gecko toe as they come in contact with a “smooth” surface:
Just one seta doesn’t produce much pull, but the combined attraction of thousands of them is enough to let the gecko climb up walls and hang upside-down from the ceiling.
Plus, if the gecko doesn’t want to stick to something, it doesn’t have to. Only when the toes are pushed down in a certain direction are the setae able to spread out and create a strong hold. But once it’s attached, it’s still easy for the gecko to get unstuck. While the force created by the toes opposes gravity easily, it isn’t so strong when pulled at a different angle. Geckos can manipulate their toes to peel them off at this angle. This directionality also keeps the toes from picking up debris and getting dirty.
Scientists are now trying to steal the gecko’s trick for use in our everyday lives. By creating artificial setae like those on gecko toes, researchers think they can create a whole host of useful and interesting technologies: gloves that let you climb buildings or never lose your grip on a football, feet that would allow robots to walk in places that are dangerous or inaccessible for humans, wound dressings that will stay on underwater.
And yes, even a reusable, super-strong tape to replace the screws that hold your furniture together. Push in one direction to stick your furniture together, pull in another direction to take it apart. Instead of fumbling with a screwdriver during your next big move, all you’d have to do is peel off the tape holding everything together. We may never be able to walk across the ceiling unassisted, but if geckos can make moving furniture easier, we’ll be indebted to them—and to the clever scientists and engineers who study them.
About the author
Andrew McAllister is a fifth-year doctoral candidate in the University of Michigan Applied Physics Program. He studies light-emitting diodes (LEDs) to improve their efficiency and make LED bulbs affordable for everyone. But he never dons a lab coat or makes an LED bulb himself – he simulates everything on supercomputers. When not sitting in his office, you can probably find him dressed in spandex (on a bike!) or buying books at Literati. You can connect with him on Twitter or his website.
Read all posts by Andrew here.
Figure 1: Andrew McAllister
Figure 2: PubMed Central