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Diffusion – or why a plastic bucket full of water does not just get bigger because of the water pressure

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Nature does not like differences in concentration in a liquid or a gas. Since the atoms all move at temperatures above absolute zero, the concentration differences are compensated for until everything is distributed evenly without external intervention. 

This process is called diffusion. Diffusion also occurs with solids, for example, the water molecules penetrate into our plastic bucket, the plastic swells up until it is saturated. It is full when it no longer likes new molecules. 
I know from schnitzel eating, that a certain amount is possible, but then it is enough. 

If a plastic component falls out of the injection molding machine, the plastic is still hungry for water. In this dry state, the material is brittle, stiffer and stronger. If the plastic component has been lying around for a while, it absorbs water from the surroundings and it swells. As we do with too much schnitzel. 
But what also happens: the elastic modulus of the plastic becomes smaller and the strength values ​​decrease. And tension arises! 

You should take these aspects into account when designing plastic components, or at least you should know of them. 

In addition to mechanical loads, plastic parts are also exposed to environmental influences. These environmental influences can be caused by chemical interaction with the contacting media (fuel in the tank), by radiation (e.g. UV radiation) and temperature. The advantage of plastics is that they can be designed with additives according to the stress. 
So, we can determine how much schnitzel can fit in. 

But how can something like this be calculated? 

The universe is sometimes complex and sometimes simple. Diffusion behaves like heat. The driving factor is now the concentration instead of the temperature, the heat capacity corresponds to the saturation value and the coefficient of thermal conductivity corresponds to the diffusion coefficient.

Video 1 swelling process of a box due to diffusion 

And now we can use a standard FE solver to calculate what our bucket does. We find the necessary values ​​in the literature. It is nice when a trial shows that we calculated correctly. 

If you would like to know more of the technical aspects, please contact Dr. Brehm. 

A lecture by him on this topic was unfortunately canceled by the organizer due to Corona. So, he is happy if he can pass on his knowledge to you. 

I am only responsible for the rough. 

Yours, 

Stefan Merkle 

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