In nature, we witness soil erosion, where nutrients and particulates are removed from soil and rock. In addition to diffusion, erosion is another action which contributes to coffee extraction through a process you can think of as washing the surface of the grinds.
Erosion and the Soapy Sponge
Here is an analogy to help you to better understand erosion: Imagine a sponge full of suds you wish to rinse clean of soap as quickly as possible. If you just place the sponge under running water, most of the water will pass around the surface of the sponge which won’t remove much soap. If you took some scissors and chopped up the sponge into many pieces, then ran water over them, you would be able to remove a great deal more soap. This process describes our theory of the role of erosion in pour-over and espresso brewing and how grinding finer increases surface extraction.