Science Questions

What does salt do to Silly Putty?

Tue, 28th Jul 2015

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Question

Ed Wilson asked:

What effect does salt have on Silly Putty? And could you really cover the Earth when the putty wouldn't flow up slopes?

Answer

Ewen Kellar tackled this question from Ed Wilson...

Ewen - OK well you’re quite right, and we’re not claiming that the Silly Putty  would Saltroll up the hills and certainly any bump, or tree or a plant or a building is going to act as a bit of a barrier and stop the Silly Putty enveloping it. But I think close to where our 17 kilometre square cube does touch the earth, I think it will probably envelop most things at that point. So I think, yes, gravity will take its toll but I think initially we will get a complete deluging of this crazy liquid.

Chris - And the salt or sugar?

Ewen - Probably the easiest way I can describe this is by giving you an idea of why the Silly Putty works or similar types of materials work. So basically these types of fluids, these non-newtonian fluids work in a way that the molecules interact with one another. These long chains of essentially polymer type molecules which, sort of, partially lock together. This is a phenomenon called Hydrogen bonding where you have an interaction between Hydrogen and Oxygen groups on one molecule which lock on to the opposite Hydrogen and Oxygen on another molecule- essentially makes them sticky, makes them start to lock together. And if they’re made to stop in a dynamic way, say if you hit this liquid with a force, with a hammer, you will cause it to react like a solid and that will give this weird phenomenon of cracking on the surface. If you add something like salt to the material, the salt dissolves into the Silly Putty and actually stops this sort of interaction happening.

Now I can give a real example of my discovery of non-Newtonian fluids when I was probably about 8 or 9 years old and that was when I was trying to make custard on my own.  So we are talking about  the old style custard powder, not the stuff which is all mixed up. So here you took your custard powder , you put it in a bowl and you were supposed to mix in an amount of sugar and milk all in one go. But what I did was I mixed the milk in first and what I discovered was this non-Newtonian fluid behaviour. Because essentially custard powder is just cornflour which is essentially starch with some egg powder in it. But when you add the sugar into this amazing liquid it stops being an amazing liquid and just becomes a boring one. So here we have exactly the same scenario where the sugar is dissolving in the milk, which in turn is then disrupting this phenomenon and stopping the Silly Putty or custard  to work.

Chris - So it becomes Un-Silly Putty , Ewen thanks very much.

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