I'm trying to better understand the chemical reaction mechanism of sulfuric acid on glass surface. I've learned by acid attack small amounts of (mostly mono- or divalent) cations are leached out from glass surface. I'm using 50% sulfuric acid solution to achieve different levels of roughness. I'm curious why alkali dissolution causes bubble formation on glass surface. I thought most of the surface is silicon and randomly there are alkali domains. However, I have find AFM data showing some peaks over surface. Why do the features observed on surface after treatment look like peaks instead of valleys?

For last, I've read every chemical attack on glass involves water or one of its dissociation products, i.e. H+ or OH- ions. So I titrated all residue solutions after acid treatment during different times, but all of them showed to have the same amount of dissociated protons, even though I've reached different levels of roughness.