Coffee grains are hard to clean up

[Jame]

Inspired by the recent thread on flying coffee grains I felt the urge to ask if anyone knows why coffee is so darn hard to clean up? It doesn't stick to the rag as most substances do. I'm assuming it has something to do with the grains' surface structure, but what kind of physics describes this?

 

[alxm]

Well, your ability to pick up solids with a wet rag comes from the surface tension of the water, holding the particles in place. That in turn requires you to be able to http://en.wikipedia.org/wiki/Wetting" the surface of the particles. When things are hard to pick up, it's usually because they're hard to wet.

I think the reason coffee grains are hard to wet is because they're quite porous.