Flow into every crevice, then stop flowing
How Does Glue Hold Two Things Together?
Glue works by being liquid enough to reach into every tiny pit in both surfaces, and then hardening in place.
-
Step 1 of 5
No surface is really flat.
Press two smooth-looking pieces of wood together and they touch at only a few high points. Nearly all of the two faces are not touching anything.
-
Step 2 of 5
Glue has to be runny first.
It must flow into all those pits on both sides while it still can. A glue that will not wet the surface cannot get in, and will never hold.
-
Step 3 of 5
Then it has to stop being runny.
Some glues harden because their solvent leaves. Others harden because two chemicals react. Either way the liquid becomes a solid that is now inside both faces.
-
Step 4 of 5
Superglue sets on the water in the air.
It stays liquid in the bottle because it is dry in there. The moment it meets the thin film of moisture on any surface, the chains link up in seconds.
-
Step 5 of 5
Roughing a surface up helps.
Sandpaper before gluing makes more pits for the glue to reach into, and more area for it to hold. A polished surface gives it far less to grip.
Birch bark tar used as glue on stone tools from Italy is about 200,000 years old — made by Neanderthals, and the oldest known manufactured material of any kind.
The short version
Glue works by flowing into the tiny pits of both surfaces while it is liquid, and then hardening there.
Try it yourself
Put a drop of PVA on cling film and let it dry. It peels straight off — there were no pits for it to get into.
Topics