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The Stickiest Non-Sticky Substance

Jan 25, 2023

The Stickiest Non-Sticky Substance

- This is one of the strangest materials I have ever seen. It is not

sticky

at all. You can't even stick regular tape on it. But if I put it on top of this tomato, it holds it up, unless you turn it upside down, in which case it just falls over. Now it only sticks to the fruit? No, it will stick to a water bottle or a bag of chips, basically any roughly smooth surface. And that's because this material is made to mimic a gecko's skin. Artificial gecko skin comes out of Professor Mark Cutkosky's Stanford lab and has now been used in robotic grippers, in little robots that can pull much more than their weight, in a robot that floats around the International Space Station, and even that allows a person climbing a glass wall, Spider-man style.
the stickiest non sticky substance
It all started with a competition to make a robot that would climb a vertical wall without suction. - Here's the

sticky

guy. Everyone is familiar with it. You press it and it sticks. He sticks pretty hard actually. And then you peel it off. Here is gecko material. And the first thing you notice is that it's actually... It's not

sticky

at all, but if I lift it out of the middle I can lift a football. You could say, "Well, you could do that if you had this other duct tape," right? Well yes, but if I had another adhesive tape I wouldn't do that.
the stickiest non sticky substance
And that's important because if you're a little robot trying to climb a wall, you can't afford to have something like gum on your feet, because then every step you take is an effort. You want something that only grips when you need it to, and that's the main principle of our gecko-inspired sticker. Here is an early prototype of the gecko-inspired

sticky

robot. This led us to a long effort to understand what makes gecko sticking work, having something that grips but isn't

sticky

. - Geckos are incredible climbers. They climb walls and we even walk on ceilings, but for a long time I didn't know how they did it.
the stickiest non sticky substance
They do not use hairs or spikes like spiders or other insects. You might think they would have little suction cups on the end of their feet, but their stickiness is actually much stronger than suction cups. - A gecko can easily hang its entire weight from just part of a toe. - The hint of how this works and can be seen if you zoom in on a gecko's toe. These are called sheets. They are regions of stems, which are called setae. And the mushroom branches into these incredibly fine structures called spatulas, which are less than a micrometer wide. - And that's important because the physical principle they use to hit each other is actually incredibly weak.
the stickiest non sticky substance
It's not like an ionic bond where the atoms are attracted to each other because one is positive and the other is negative. It's not even a hydrogen bond like in water where part of the molecule is slightly positive and the other part is slightly negative. No, geckos rely on the attraction between neutral atoms. So how does that work? Let's say a gecko is climbing through a glass window. The gecko's atoms are neutral and the glass atoms are neutral, but at any particular instant the electrons around the atom are not perfectly evenly distributed around the nucleus. They may be a little more on one side than the other.
the stickiest non sticky substance
And this makes the atom momentarily a little positively charged on one side and a little negatively charged on the other side. Now if there is a neighboring atom very close to it, say an atom for glass is within a few nanometers, this charge can induce a complementary charge imbalance in the glass atom. So now the electrons from the glass atom are attracted to the nucleus of the gecko atom and vice versa. There is a very weak attractive force between the gecko and the glass. And this is known as a Van der Waal force. Van der Waals forces are at least partly why gage blocks, smooth, flat pieces of steel, can stick together. - It's there all the time.
the stickiest non sticky substance
If you have a car with, you know, you can put these vinyl sheets on the rear window so your kids don't like excessive sun when they're in the car seat. That is also the Van der Waal Force. - But we usually don't notice Van der Waal Forces. We don't feel them with our fingers because, at least on a molecular scale, our skin is incredibly bumpy. Touching a piece of glass is like putting a mountain range on top of a plane. There are not many close contact points. Geckos get over this by using all those little branches. - So this means that when the gecko puts its foot down, it has a large intimate area of ​​contact.
It's almost like you pour glue on the surface and let it flow. And that's what makes adhesion work. - It is currently impossible to replicate the intricate branching structures of the gecko. - We can't do what the gecko has. we can't - That structure, that really fine branching structure. - That fancy branching structure. but we can make more crude approximation. - Under a microscope, you can see that the artificial gecko adhesive is covered by rows of sharp wedges. The tips are about one or two micrometers wide. That is a hundred times narrower than a human hair.
Creating such fine structures requires a laborious process. A block of wax is used as the base for a mold and then a razor blade is repeatedly pressed into the block creating wedge-shaped indentations. A silicone polymer is poured into the mold and a backing material is attached. - Is this silicone? - So it's called Sylgard 170. And it's a type of silicone, yes. - And after about 24 hours, the adhesive is cured and ready to use. And that's what it looks like. But the mold can only be used a few times before the quality of the adhesive declines and you have to start over. - This sharp wedge structure, which has this interesting property that the first time you bring it to a surface, the only parts in contact are these points.
So therefore it is not

sticky

because there is no Van der Waal Force worth mentioning. As he loads it into slice, all of these things double up and we get a much larger contact area, almost continuous in fact. And just like the gecko, we have some Van der de Waal forces to work with and that is what gives us adhesion. - So, to make the adhesive stick, you have to pull it parallel to the surface, that's known as the shear force, and you have to pull it in the direction that will bend the shims so they make contact with the surface.
If you pull it in any other direction, it won't stick. This makes it easier to remove the adhesive from the surface. I mean, there's basically no contact force. - And so the way we clean this is just with tape, because the tape doesn't stick to the Sylgard, but the dust particles do. - They used this property to create an incredibly small and light robot called the MicroTug. It weighs only 17 grams, but is capable of pulling a weight of 20 kilograms. That's the equivalent of a human towing a blue whale. Only six of these little robots can tow a car.
The gecko sticker is on the bottom of the MicroTugs. So when they are pulling the car, the material cuts through and sticks firmly to the ground. But as the car moves forward, the shear force decreases and therefore it is easy for the robot to get up and move forward. The amount of pulling force achievable depends on the area of ​​adhesive that is in contact with the surface. So they've developed a method to measure this by shining light through the acrylic. - So what it does is it shines the LED through the acrylic and every time there is contact with the surface, it thwarts the beam and shows you exactly where the contact area is. - One square inch of contact area can support the weight of about four and a half kilograms or 10 pounds.
Since the adhesive is directional, to hold an object you need two pieces of adhesive attached in opposite directions. So when you pull up, both sides are in cut and both stick together. Actually, two opposing pads of adhesive were tested on a robot on the international space station. - His name is Astrobee. Think of it like a drone, a UAV, that really just uses a fan to propel itself inside the International Space Station. And the idea is that it could be there to take videos or look for something for the astronauts. Our proposal was, "What if it should be able to float smoothly and stick to a wall, for example.
Or pick up a big box." - Our goal is to make the blue LED turn green, because there is no gravity. You don't really need a lot of force, but you want to be able to grip very smoothly and release easily. So we got the gecko stickers on the International Space Station and they work just as well there as they do down here. - This went to space, this was on the space station? - This was on the space station for about a year and a half.- Wow. This principle can be extended to three pieces of adhesive so that when you pull them all stick to a flat surface.When at rest these things stick out a bit and then , when you pull there, they become flatter, right? - Yeah, the whole pad doesn't touch because it's both surfaces.
It's not perfectly flat. And then once you activate it. - Oh yeah, I totally could see that. - So I'm going to turn off the power so there's absolutely no power at all and it's still attached to the surface. - Yes, I was thinking that you would have to keep the engine running, but is it something like locks in position or what? - Yes, as long as there is some tension, it just stays there forever. - There are many potential applications for this material, but the most obvious is for robotic grippers. Since the adhesive adheres well with only a small shear force, it is excellent for picking up delicate items such as produce. - These are meant to be smart gecko palms, basically.
So we're trying to understand with a minimum of compression. - We're not even really involving the ratchet mechanism there. - Yes, not at all. - So it's like less than a Newton. You can also grasp bulky objects or palm a basketball. Now can the gecko sticker pull a car? They've hooked him to this rope, which is attached to a winch here, but the winch isn't really secured. It's attached to four pieces of gecko adhesive that are just sitting on this pipe. So the question is, can these four pieces of gecko adhesive anchor the winch as it pulls that car back? (winch screeching) Oh yeah, it's moving. (winch screeching) Wow.
Could you take this out? - Yes Yes. - How much does it cost... as if nothing was holding it down? That's wild. But of course the app that everyone wants is to be able to climb a building. And this was actually a PhD project for graduate student Elliot Hawks. It was a challenge because he had to get enough gecko adhesive in contact with the glass at any one time to support his full weight. Let me know if you want me to try climbing a building with

sticky

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