percolation
"A large part of the intense fundamental interest in percolation is due to the fact that percolation is like a phase transition, in the sense that there ..."
"The ball moving through the pins is similar to water moving through a solid medium" The video is a pretty good visual display.
The movement of water or any other small molecule through a polymer is kinda like the movement of small meatballs in a big bowl of spaghetti. In this here analogy, the meatballs are water and the spaghetti are the polymers. This analogy maybe only relevant for polymers.
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