Spider-man’s synthetic web fluid is described as “a shear-thinning liquid” that “on contact with air, the long-chain polymer knits and forms an extremely tough, flexible fiber.” Is this a material that sounds realistic?
Absolutely. Shear thinning means it is originally a stiff or highly viscous material, but if you shear it, it becomes less viscous and can be easily aligned in the shear direction. That’s why you create a certain kind of alignment of molecules.
For example, ketchup is a shear thinning material. It’s difficult to get out of the bottle, so you shake it to shear the molecules in the ketchup, which decreases the viscosity. The biopolymer chains will stretch so they become disentangled and slip away from each other, and that’s why they can come out of the bottle.
In 3D printing, shear thinning is very important because when you print, you want the material to go through as a liquid, otherwise it will clog the nozzle. But once it comes out you want it to be solidified, immediately, otherwise the material collapses, and you won’t be able to have a good print.
One of Spider-Man’s powers is the ability to climb walls and buildings. There are a few animals in addition to spiders, such as geckos, who can also do this. How does it work?
In the field of robotics, the gecko excited people because their ability to climb walls is not based on the capillary or on the vacuum. With frogs, they have liquid coming out, so it’s kind of sticky and messy, plus you would have to carry the liquid for the robot. Likewise, if it’s based on a vacuum, you need to carry the vacuum pump, so it’s not energy efficient.
I showed this movie in my class: This guy was using vacuum pads to climb, and even though it’s just a five-level building, it takes him several hours. At the end he saw rain and he started to worry, because if you have water your vacuum doesn’t work anymore.
That’s why people are interested in the gecko because it has none of this. It’s based on the Van der Waals interaction through microscopic hairs, called setae, on their toe pads and at the end of each seta there are about 1,000 nano hairs, called spatulae. They have many, many rows of setae. When setae/spatulae are straight, there’s very little contact of them with a flat surface, so it’s very easy to come off. However, when the setae bends, the contact area increases significantly, considering there are millions of spatulae, so you have better adhesion. The setae can open and close this way, so that’s how they change the adhesion.
We can fabricate those structures that can mimic the structural adhesion, but the problem is that the gecko is only 50 grams and they have millions of these setae on their palm. It’s actually over-engineered. A human is at least 50 kilograms, so 1,000 times bigger.
For comparison, super glue adhesion is about 1,000 newtons per square centimeter strong, duct tape is 50, and the gecko setae is only about 10. So researchers face this dilemma: If you want super strong, like superglue, it’s not reversible, and if you want it reversible, you can’t go super strong.