By Sam Cadwell
If you’re a fan of comic superheroes, some suspension of disbelief is required. The Official Handbook of the Marvel Universe explains the amazing Spider-Man’s ability to climb walls as his ability to “enhance the flux of inter-atomic attractive forces on surfaces he touches, increasing the coefficient of friction between that surface and himself.” This statement is meant to sound scientific, no doubt, but actually makes less sense than if Spidey climbs walls the way real spiders do.
There are a variety of weak intermolecular forces that arise between molecules a nanometer apart or closer, often collectively called van der Waal forces. Such forces are part of what determines the coefficient of friction between two surfaces. Though mass is used in calculations of friction between surfaces, coefficients of friction are not dependent upon mass but are qualities of a material dependent upon system properties like temperature, velocity, atmosphere, and the geometry of the interface causing friction. Exactly how Spider-Man could “enhance the flux” or alter the flow of van der Waal forces is unknown; even electrostatic “super powers” capable of inducing localized dipoles in skin cells would do little for sticking to walls as a pair of gloves easily puts said skin cells more than a nanometer from any vertical surface and thereby outside the range of van der Waal forces.
If disbelief is to be suspended, a more plausible mumbo jumbo explanation must be provided. So how do real spiders do it? The answer lies in increasing the surface area of the interface, not in increasing the coefficient of friction. Spider feet are covered in over 600,000 setae, or bristle-like hairs, with microscopic triangular barb ends. These hairs increase the amount of surface areas in contact a nanometer or closer, allowing the collective van der Waal forces to greatly exceed the weight of the spider. How does a spider unstick? By slowly peeling each foot off so the force required to overcome the adhesion between the setae and the wall is minimal (Kesel et. al., 2004). The burning question is, could this work for a man-sized creature?
A human would probably have a pretty difficult time climbing like a spider. We’ve only got four limbs and we’re rather impatient so peeling each contact point off slowly along the length of a wall might take a little joy out of the wall-crawling. However, fear not! There’s another creature who sticks to walls like a spider does whose mechanism is similar but better suited to human bulk: the gecko. Geckos also have setae on their feet, but nature has invented a fantastic yet simple way for them to control their adhesion. Rather than having triangular barbed ends to increase contact points, gecko setae are microscopic flexible strands that branch into even tinier spatular tips and bend to contact to surfaces sideways (Keller et al., 2000). This directional adhesion means the force of gravity keeps a gecko’s foot locked against a surface via the cumulative van der Waal forces of the bristly setae sides, but lifting its foot in a different direction easily breaks the weak bonds. To travel in different directions, geckos have rotating ankles. Watch one climb upside-down a wall and you’ll notice its feet are still turned right-side up.
An adhesive based on physics rather than chemicals is pretty cool. Research into practical applications based on gecko adhesion is underway. If you want to learn more, check out Stanford’s Stickybot (III) and some articles about potential uses for this technology based on nature’s design. Personally, I’m waiting for more results from project Z-man: Stanford’s Spider-Man suit. It’s been a few years since they’ve mentioned the project in the news, but I remain eagerly optimistic.
Spider feet info Kesel et. al., 2004 doi:10.1088/0964-1726/13/3/009
Gecko feet info Kellar et al., 2000 doi:10.1038/35015073
More on practical applications: http://www.bbc.com/news/science-environment-19875247
More on Stanford’s Stickybot: http://news.stanford.edu/news/2010/august/gecko-082410.html http://singularityhub.com/2010/09/10/stanfords-stickybot-is-climbing-faster-human-project-coming-soon-video/