Eli5 why does cling film sometimes stick perfectly to some plates/bowls but other times it does not?

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Eli5 why does cling film sometimes stick perfectly to some plates/bowls but other times it does not?

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It often depends on the surface porosity and/or imperfections on the bowl or plate you’re trying to cover. Temperature also plays a role as higher temperatures will cause the plastic to soften and become more tacky. It’s intersesting to note that these wraps used to be made of PVC however it was determined that the chlorides were not healthy. Now most wraps are made of LDPE (Low-Density Polyethylene) because it is considered to be safer for the body but it results in a less effective cling. So yes, cling wraps used to work better than they do now.

If you Google this, you get all kinds of answers. The obvious one is how clean the dish is. The cleaner the better.

But the real answer is how smooth the dish is. If the dish has a very smooth and polished look, cling wrap clings better. A matt finish is bumpy at the microscopic level, making less contact with the cling wrap.

And you might say “sure, but why would that matter?” The answer to that is that much of the cling in cling wrap is an electrostatic cling, and only works well with very close touch. A bumpy texture holds the plastic above most of the surface, only touching the tops of the bumps.

There is a small but real chance you are using cling wrap on a metal dish, and one that is very smooth. What gives? This often does not work because the metal discharges the electrostatic charge. No static, no cling. Glass works well because electricity does not travel easily through it, preventing electrostatic discharge.

You know how it’s harder to pick a coin or card off a clean, smooth table than a dirty, rough one? It’s the same with plastic wrap.

I think cling wrap works much like a suction cup would work when clinging on a smooth surface. The cling wrap when wrapping on smooth surfaces creates on that smooth surface areas with lack of air. As you add tension on the cling wrap as you wrap it around on let’s say the sides of the lip of a bowl, suction is created that counteracts this tension. These two forces that counteract each other makes the cling wrap stable, letting it cling onto the sides of the bowl.

What about a surface that is not smooth? A rough surface (even just matte surfaces) on the other hand has very small bumps that does not allow the areas lacking air to be maintained. These small bumps push on the cling wrap creating mini entrances for air to move into the area the cling wrap is supposed to cling onto. So, as you pull onto a cling wrap touching a rough surface, a low pressure is created in between the cling wrap and the surface it is touching. Air rushes into the area where the cling wrap is touching as you try to put tension into it thus breaking the seal.

This is just my speculation.

Some cling film actually has glue attached to it. As you have probably seen, glue sometimes doesn’t stick very well to things that are greasy or slippery like smooth plastic. Another way cling film works is by stretching and then folding the film over the top of a bowl. When you stretch the film, it tries to pull back. but the film is folded over at the edge of the bowl and stuck to the sides. It doesn’t have to be stuck very tightly to the sides in order to hold the stretchy part going over the top of the bowl. You may have noticed this same thing when pulling an electrical cord that gets stuck when you go around a corner. It doesn’t take much “stickiness” to keep something stuck when it changes direction.