How does Blu-tack work?

23 March 2009

Question

Why is it that Blu-tack is very good at sticking one thing to another when it itself doesn’t feel that sticky?

Answer

Chris - I think there's two aspects to this. One is in the same way as the water gets in to all of the nooks and crannies both in the skin and the surface that you touch - in the same way as when you lick your finger to turn a page - you create more friction between your finger and the surface. The water forms an attachment on that surface. Blu-tack is plastic. In other words it can deform plastically. It gets into the nooks and crannies of the surface you're sticking it to and the surface it's already stuck to which makes it sticky. The other point is that when you press it onto a hard surface the Blu-tack forms a very smooth, flat surface against that hard surface. This excludes air. In order to get the Blu-tack back off the surface you've effectively got to break a vacuum. The atmosphere is helping to hold the Blu-tack stuck onto the surface. That's why I think it's sticky. Ben - That's the same reason why, if you get two sheets of glass and layer water between them and press them together to exclude the air it's really hard to peel apart even though water isn't sticky at all. Chris - Two microscope slides are almost impossible to separate if you put a drop of water and then put the two glasses together. You've got to twist them off each other and you can't pull them apart because the atmosphere is squeezing down on you. Every square metre is feeling a force of ten tonnes: the weight of a London bus. On our bodies every square metre of our body's got ten tonnes of atmospheric pressure on it. That's what's holding your leaflet stuck to the wall with Blu-tak.

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