What makes it so that coffee doesn’t separate into water with coffee particles resting at the bottom?
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“Coffee particles” aren’t really a thing. Coffee is water with a whole bunch of other stuff mixed in. Most of that stuff is water-soluble, which means that its molecules will break apart and stick to the water molecules. An example of something else like this is salt. It will dissolve into water and spread out.
Soluble molecules can become insoluble as the water cools, and form a powder in the water that eventually sinks or an oily layer on top.
Coffee isn’t a suspension, small particles in a fluid. Coffee is a solution, several liquids dissolved in an overall fluid (water). In general, solutions don’t precipitate out absent some chemical reaction.
Coffee is essential a beverage made of solubles from dry, ground coffee. Roughly 20% (so they say, I’m only repeating what I learned at Intelligentsia), is insoluble, meaning fibrous matter that doesn’t attach to hot water as you make coffee.
The other stuff is. During the brewing process, certain solubles unlock from the cell walls of coffee (caffeine, acids, lipids, carbohydrates or sugars) at different parts of the brewing process.
So what you have left is a mixture of these solubles and the brew water.
However, leave a pour over for a few hours on a table and you will noticed some “sediment” on the bottom. The liquid will look unchanged but some particulate can settle.
The filter does. All the solid coffee particles get caught in the filter, so only the liquid coffee can pass through
Most particles are filtered out, the coffee that we drink is actually soluble (dissolvable) things from the beans that are dissolved during the brewing process (it happens in all of them, from cold brewing, to percolation, to espresso or immersion). Because it is dissolved, you can’t separate it unless you centrifuge it.
When coffee beans are ground, it releases the oils and gives access to the inner surfaces that allow brewing to happen more easily.
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