So I look at that experiment and think
--I don't know how it proves chemical or physical adhesion (meaning, it could only be one, or the other, or both, but there's no way of telling from that experiment). You would have to control for one condition.
--I would think a good test would be sand part of something, leave the other part alone. Go over with a solvent. If it softens up, it would be prone to chemical adhesion as the solids are being dissolved. If it doesn't soften, I would not think it would be prone to chemical adhesion
--I do not understand how the top crust would be different than the center of the substrate. It seems it should be a uniform layer of solids, with the same porosity through and through.