Why is it good to “rough up” a surface to be glued? Wouldn’t a smooth surface give the best adhesion?
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It likely depends on the exact type of glue being used, but typically rough is better.
Glue works by “grabbing” onto the surface, so if it was smooth, there isn’t much to hold onto.
A rougher surface increases surface area dramatically. More surface area means more bonding.
Glue grips the surface. The more surface to grip onto, the better. If your glue was unable to fill in the gaps, like tape, then a rough surface would give worse adhesion.
A rough surface increases contact area between the glue material and the item. The glue then fills in the gaps and acts as an interface between two items. A smooth surface would be best for touching another smooth surface, but you can’t get them smoothed enough and keep contaminant free in practice.
Take a glob of playdoh and push it into a mirror. Then try to take it off. Now take that same glob of playdoh and push it into a rug. Now try to take it off. Which was easier, taking the playdoh off the smooth surface of the mirror or the rough surface of the rug?
If you have two perfectly smooth surfaces – yes, that’s best. But since you typically don’t, making a uniform roughness improves adhesion. Typically, of course it can vary between materials.
Imagine trying to climb barefoot up a giant glass window VS a rock face. In the same way your fingers can grip onto a crevice in a rock, glue can grip a textured surface to bond to it.
On a microscopic level, the glue seeps into the abrasions of the rough surface and grips them better.
Like how your fingerprints help you to grip things.
I assume you’re not a parent. For the same reason chewing gum is much easier top get off, say, varnished hardwood floor than a fabric sofa.
Think of it on the molecular level. A “rough” surface as plenty of places for molecules to stick together. A smooth surface has few. This is why rough surfaces glue better than smooth ones. More points of contact.
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