Eli5: What makes things “sticky” like tape or any kind of adhesive? How do they stick on to smooth surfaces instead of coming off?
In: Chemistry
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There’s a few things going on here at once.
1. Smooth surfaces aren’t actually “smooth” the tapes’ adhesive adheres to the tape because the tape has a rough side and a smooth side. The adhesive can’t come off the tape but will peel off “smooth” surfaces. Such as itself when unrolled
2. Smooth surfaces aren’t actually smooth. Microscopically the surface is just rough but on a smaller scale. Even glass is not smooth when looked at closely. Your adhesive is actually sticking by a force called cohesion to the microscopic cracks on a surface, you just can’t see it. When ripped off the cohesion force peels off the surface but only because the same forces of cohesion are stronger in the sticky adhesive bonds at the molecular lever.
Tape is stronger than the surface it’s stuck on. Its sticking to microscopic cracks that the adhesive fills in, making the adhesive surface area far greater than you can see. So it’s a combination of surface area with cohesion forces and the tape being stronger than the adhesion to the surface it’s applied.
Regarding surface area. Ever thought why glue works ? Its just non peelable tape with strong cohesion and more surface area contact because it’s wet when applied. PVA glue dried, then peeled off your skin is a fun experiment regarding what’s being said. 😛
To my knowledge, if anyone knows better or can add more details. Please do.
Eli5: Think of glue as a really strong person, who is in between your sheets of paper grabbing on to at their molecules. A molecule being the very small blocks that make up your material, in this case paper, that you would need special tools to view.
The stronger that person is, the more your two sheets of paper will stick to each other, and thus be “stickier”.
Tape is much the same way, but instead of being squeezed out of a bottle or rolled on to the things we are trying to hold together. It comes on it’s own plastic material, so that we can have options on how we hold things together.
Eli15: Tape and most adhesives work by forming chemical bonds between both surfaces. The strength of these bonds will determine the adhesive abilities, stronger bonds, stronger stick. I believe epoxy works by filling into the pores of whatever it is you are sticking together, and solidifying into a solid mass, that locks the materials together.
So adhesive on tape sort of turns into a liquid, then it flows into the nooks and crannies of the thing you’re sticking it to, then it sort of turns back solid again and then hangs on to the thing you’re sticking it to like legos.
There’s two primary forces going on here. The first is pretty easy to visualize. The adhesive on tape is what you call viscoelastic, meaning it behaves sort of like a liquid even though it’s solid, and it’s stretchy. So it “flows” into all the oils and crannies of a surface then hangs on tight. A surface may look smooth to you, but chances are it has many pits and pores and mountains under a microscope.
The second is a little harder to visualize. All molecules are made of atoms which have electrons, which are negatively charged, and protons, which are positively charged. If an atom has the same number of electrons and protons, it is neutrally charged overall. Some neutral molecules still are negatively charged on one side and positively charged on another side, because some atoms in the molecule are greedy for electrons and pull the electrons in tight. So in these molecules, like the ones in tape adhesive, you get positive parts where the electrons went away and negative parts where the electrons got pulled in tight. Positive likes negative and sticks to it like legos, and the adhesive on tape similarly behaves almost like legos like with the surface of the material you stick it to.
Ok let’s take this in three parts:
Pressure sensitive adhesives such as tape are a mixture of rubber (like your tires) and a type of liquid that is very slow to move but still liquid. When these are pressed against a surface, they squish themselves into the cracks of the material. The liquid material will allow you to compress it and it actually moves better when squished than it does just sitting there meaning it is now stuck in the cracks. From there, it actually resists being pulled on like a Chinese finger trap. This is what we perceived as being sticky.
There are also chemical reaction based adhesives. These don’t have to be crazy things like vulcanizing rubbers, etc. This can be something as simple as honey coming in contact with your fingers. The structure of honey allows it to absorb water into its system. This means that as it contacts your finger it takes a liking to the water on your finger and gets bound to it. The only way to get it out is to overwhelm the matrix with more water (aka licking your fingers).
Finally, when it comes to smooth surfaces (no surface is actually smooth) we go back to the first type of adhesive. The idea for smooth surfaces is to not only fill the cracks, but to make a uniform surface of adhesives that can use the small gaps in the surface to anchor it to the smooth material. For the rest of the area, it is more like a suction cup than like the tape adhesive.
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Other people have commented on how adhesives are made to fill in the gaps between rough surfaces. Now, the reason they actually make things stick together is that basically *everything* sticks together. It is a fundamental property of all matter that if you put it very close to each other, it sticks together (the exact reasons have to do with atoms and electrons and that opposite charges attract).
Normally when you, say, put your hand on a wall, parts of your hand will stick to parts of the wall but the effect is so tiny that you don’t even notice. The reason for that is that both the wall and the surface of your hand are still very rough at a microscopic level, so only very few points are actually touching and there is still a lot of air left in between. When adhesives fill out these empty spaces between two objects they make more atoms be closer together, and therefore amplify this fundamental “sticking force” (called the [van der Waals force](https://en.wikipedia.org/wiki/Van_der_Waals_force)) to the point where it has a macroscopic effect.
Everything sticks to everything else when they are close together because of electrical charges in atoms. “Sticky” materials are soft enough to get really close together when you push on them.
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