What you're picturing has a name: https://en.wikipedia.org/wiki/Fritted_glass. It's used in a https://en.wikipedia.org/wiki/B%C3%BCchner_funnel instead of filter paper, to efficiently separate liquids from solids.

Also, if you take some fritted glass and wrap it around a solid core, such that the holes don't go anywhere — i.e. they're just pits that water molecules (but not anything larger) can enter, and then get stuck there by surface tension — then you've get https://en.wikipedia.org/wiki/Molecular_sieve s.

Yes, that would be a membrane. There a number of membranes with larger pores that are fabricated like above, but out of polycarbonate (track etch membranes).

Some people have also experimented with single and double layers of graphene with a few very small pores. Because they are so thin, you don't need as much porosity for high transport rates.

You still have fouling issues, though.

> If you had a thin sheet of silicon filled with 0.2nm holes

That's literally individual silicon atoms removed

"The nearest neighbour distance is 0.235 nm" https://www.princeton.edu/~maelabs/mae324/glos324/silicon.ht...

Not if it’s thick and/or rigid, right?