How is duct tape so sticky?
What force makes duct tape stick to a wall?
Chemist Kate Biberdorf got stuck into this question...
Kate - It's very sticky. And it's funny - I get this question from kids all the time. I don't know why kids want to know about this, but I am ready to answer this question. So for duct tape, it's really neat. There's basically two main forces that happen here. There's a bunch of molecules in your tape, and some have really strong adhesive forces and some have strong cohesive forces. So 'adhesive' is when something sticks to something else. And so that's the part of the tape that's going to allow your tape itself to tape to the poster, and then also tape to the wall. The problem is you need the tape to stick to itself as well. So that's the really cool part. That's the cohesive part. So when a molecule wants to touch just itself, stick to itself and not touch the wall at all, that's going to be its cohesive properties.
Chris - What actually chemically is going on to give the molecules on the white side of the tape - that are really sticky and pull your fingers apart when you try and get them off your fingers - that actually gives them those adhesive properties?
Kate - So they're called intermolecular forces. And so it's when a molecule wants to interact with another molecule physically, without going through a full-on chemical change. We're not having an explosion here or a bomb, we're just having some intermolecular forces. So usually it's like the negative side of one molecule is attracted to the positive side of another molecule. It's kind of like when you see a hot guy across the bar and you're like, "ooh, I like that". And you just walk across the bar, you just go straight over there. It's the same type of attraction. And so you're coming together, maybe you hold hands for a moment. Maybe you kiss for a second, but you don't chemically change, your body doesn't change. So you're just going to hang out and you're holding hands. But then when you want to rip your poster off the wall, you can just let go of their hand and move on and go on with your day.
Chris - You're a fast mover, Kate. They do things differently in Texas. We know we're a bit more sedate here in the UK. We take our time with these dates.
Kate - We go after what we want here.
Chris - But you can probably anticipate where I'm going to go with the next question, which is, you mentioned about cohesive forces. You said the lovely line, "you need the glue to stick to the tape too". This is going to cue the question in everyone's mind; if Teflon is so slippery, how do they make a stick to the pan?
Kate - It has different sides of the molecule itself. So you can't always assume that one molecule is a hundred percent uniform. Just like we have like long hair on our head, we don't necessarily have hair on our feet. Molecules are kind of a little bit different as they're asymmetrical. So maybe like the head of the molecule is able to attract to the pan itself and that's what's clinging onto the pan for dear life. And then the feet itself gives like the repellent property. And so that's what allows for your food not to actually stick on the pan because it's repelled from that side of the molecule. So it's all about asymmetry.