How does glue work?

Andrew - I mean, glues are sticky only really when they're connecting two surfaces. It's not a kind of magic property that they have. And they only stick when the right surfaces and the right glues meet each other.

Chris - Hence the old joke, you know, how do they get the glue to come out the tube then if it's so sticky, why doesn't they stick to the tube?

Andrew - Exactly. We've all probably got examples from our woodwork classes of glues that never worked and so on. And it's back to this story about the forces between molecules: adhesive forces and cohesive forces. Because if you look at two surfaces in contact through a high powered microscope, it is extraordinary to see that they're really, really rough. And it's like this kind of top of a mountain range on one surface meeting the top of a mountain range on another surface and grating against each other. So actually, when two surfaces are in contact, it's mostly empty space in between them and just the peaks, one peak meeting another peak where they're actually physically in contact. So the job of glue is to get in there into the space between two surfaces. And it might penetrate through cracks or it might just get absorbed into the surrounding material, or there might even be a chemical bond. But once the glue penetrates one way or another into the two surfaces, that would be bonded. The question is, do the forces that hold the glue molecules together, that's called cohesion, are they less or greater than the forces of attraction between the glue molecules and the surface, which is called the adhesion. So glues have to adhere from the glue surface to the surface they're connecting, but they also have to cohere so that the glue doesn't fall apart itself.

Chris - Can you help me out then, and tell me why I can't get Weetabix off of a bowl that's gone dry.

Andrew - <laugh>. Well, there's a new one.

Chris - I mean, it's a similar thing to wallpaper paste, I presume, isn't it? Because it's starch. And I presume because that's a stringy molecule. It's doing exactly as you say, and threading the tendrils of starch into the rough surface of the ceramic in the same way as the wallpaper paste gets into the back of the wallpaper and the wall.

Andrew - Very good point. And at the fundamental physics level, it's electrical attraction. To make it simple, it's the fact that one molecule electrically is attracted to another molecule.

Chris - I have a lot of electrical attraction. I'm sure you can sympathize with that.