Part of the show Science Q&A Show
Mike in Moulden asked:
I often use silicone sealer, the kind of stuff you put round the bath. The only thing that seems to finish it off into a right angle (ie: right into the corner of your bath so water doesn't drip behind it) is to wet your finger. Over the years I've tried all sorts of things including a piece of round steel, fairy liquid, salt water and everything. But the only thing that really works is to wet your finger and keep running it along. Why does that work?
Silicone is a big polymer, which is a bit like a load of spaghetti chains all linked together. It's lots of silicon surrounded by oxygens, and they form these big long chains linked up end to end. They are actually water repellent. If you lick your finger and smooth it, then what you're able to do is push it into place but it won't stick to you because your finger's got a layer of water on it and it doesn't like water. This keeps the silicone away from you and as a result you can push it into the shape you want. As for why it works with saliva and not salt water or ordinary water is that there are other clever things in saliva such as a protein called mucin, which is why your saliva's all slimy. That may also be helping to lubricate the silicone from locking onto your finger. That's the only thing I can think of. Perhaps someone else can help us with this!