What makes things sticky?
Dear Naked Scientists, My name is David Zucker, and I'm a long-time avid listener. I've wanted to write in for ages. You've even covered an article that I co-authored (Alavez et al. Amyloid-binding compounds maintain protein homeostasis during ageing and extend lifespan. Nature 472, 226-229 (2011)), which was quite exciting for me. I have a question of my own: What makes things sticky? I've always wondered what makes materials like glue and tape sticky on a molecular level. What is going on chemically that causes my fingers to stick together when I have honey on them? I know you've covered the gecko phenomenon, but I hope you can tell me other sorts of sticky stuff.
Thanks for all the time and work you put into producing a great show!
Your friend from California by way of Tel Aviv,
Philip Broadwidth - Well David, that's a very good question. Basically, there are two kinds of things that you could think of when you're talking about stickiness. When you're talking about things like glue, sort of a super glue, there's often a chemical reaction going on and when you're talking about things like honey, it's not an actual chemical reaction but just interactions between the molecules. So, for example, super glue is made from something called methacrylate and in the bottle, that's fine, but when it comes into contact with water either in the air or on your fingers, then it starts a chemical reaction which bonds all of the molecules together into big long chains which is what sticks everything together, then the chemical bonds hold everything together. Same for lots of other kinds of glue, but things like sugar or honey which just feel kind of sticky, that's generally down to forces that are not actually chemical bonds, but interactions between molecules. So, some of those would be hydrogen bonds which are the same interactions as between water molecules, but a sugar has lots of sites that can hydrogen bond which means that there's a more cooperative effect. It's like putting lots of hooks into a surface and then trying to pull on it. If you've only got one, it's relatively easy to pull away, but the more hooks you have all acting together, the more sticky something is.
The other alternative to that is something called Van Der Waals forces which are generally to do with the surface area, it's a charge interaction. Things like gecko's feet have exceptionally high surface area which is what allows them to stick to glass for example. But that's also the same forces that are in play when you look at things like post-it notes. They have a material on the back which is in very small spheres. When you press it down, that increases the surface area and allows it to stick to the paper, but it's not such a strong interaction that when you peel away the post-it note, it leaves the paper that you'd stuck it to intact.