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Walking on custard

David Gregory | 16:22 UK time, Tuesday, 8 September 2009

Other brands are availableIt is perhaps one of the most famous experiments done by the Brainiac team. Lets go to tape. But to help launch this blog can we recreate their famous walking on custard experiment?

First of all why does it work? Jump in a bucket of water and the water splashes everywhere obeying Newton's third law. Every action has an equal and opposite reaction. So the action of the jump gives us the reaction of the splashing water. But luckily for us not all liquids are Newtonion.

Custard, quicksand, ketchup and emulsion paint are non-Newtonian liquids. Which means they don't behave like water. If you apply sudden force to them they either solidify (custard and quicksand) or liquify (ketchup and paint).

A large vat of custardWhy? Well in the case of custard think of what goes on when someone tries to cross a very crowded room. Moving slowly through the throng is easier than running at the group at high speed.

In the custard the energy of the impact causes the starch to act more like a solid than a liquid.

This means in theory you can walk on a swimming pool of custard... or given BBC budgets jump up and down on a small bucket of it!

Handling a non-Newtonian fluidSo that's what we're trying here. It really does work. More than that I can't quite believe as a scientist and now science reporter that I'd never made a non-Newtonian fluid before. They are fascinating fun.

For parents or children who'd like to try something on a smaller scale in a plastic bowl at home then I recommend cornflour. It's cheaper, and more importantly won't stain your hands or even feet a bright yellow.

UPDATE Some people have asked for our recipe! To fill our plastic crate to about ankle height with a cornflour mixture took 10 boxes of cornflour. Each containing 500g. We poured three boxes into the crate to start and then added water before mixing by hand. You are aiming for a thick paste. You've created your non-Newtonian liquid when you try and get rid of apparent lumps before working out they are actually caused by the paste solidifying under the stress of mixing! After that it's a question of adding more cornflour and more water to make up as much as you want. So far our crate has lasted two days with a small amount of separation which is fixed by more mixing.


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