Twenty-first Sunday Salon 11.19: How to make quicksand

One of Hodmandod Minor's friends gave him HOW TO FOSSILISE YOUR HAMSTER for his birthday and on page 161 is a recipe for making a dilatant material from cornflour and water AKA 'Magic Custard'. The recipe is 300g cornflour to 250ml of water, and you simply mix them together with a spoon in a bowl until it becomes too hard to move the spoon. Then you tip the bowl to one side and stir again.


The mix has the same properties as quicksand in that when you stir it...


it becomes solid, and when you stop it becomes liquid again.


Just like quicksand. If you put your foot in gently it will flow like a liquid around you, but try to pull yourself free and it will solidify. It is quite fascinating. For instance if you plunge in your hand quickly you can scoop up a ball and mould it like clay...


but if you leave it there it will soften


and then flow through your fingers.


A spoonful will flow and also drop from a spoon as if it is undecided - liquid or solid?


And if you plunge in your hand quickly and lift you hand into the air the bowl will rise too, just for a while until it remembers what it is: liquid.


The reason for this strange behaviour is that each of the particles in the cornflour is surrounded by a limited number of water molecules. Normally they are randomly arranged, and a gentle force allows them to slide over each other so the bulk properties are like liquid. However, if a force is applied, like a spoon, a hand, or a foot plunged in, the cornflour particles are momentarily forced to arrange themselves into an ordered rigid structure (because there are so few water molecules around to allow much movement), and so they behave like a solid - until they have time to become disordered again.

Governments are interested in developing such materials (shear-thickening fluids STFs) to make body armour - something that is normally soft and easy to wear, but that will stiffen and protect when a sudden force, like the impact of a bullet or a knife, starts to be applied. But since the same thing would presumably happen with other sudden movements, I forsee unfortunate consequences.

Interestingly, when cornflour is boiled with water the grains of starch burst open to release long molecules of starch to form custard which has opposite properties: when you stir custard it becomes more runny, but when you leave it alone it sets into a solid (thixotropic). This is because there are lots of weak bonds between the long molecules so they link together to form a kind of 3D mesh - but when this mesh is stirred the bonds are broken.

I have always loved stuff that can't make up its mind: graphite that is both metallic (conducts electricity) and non-metallic (dull and weak); wax that can flow or break; rocks that look like any other sort of rock in daylight but in the dark glow as if they have become scraps of fire; liquids that mixed together cause a solid to form or suddenly become icy cold; and once I cooled some ammonia until it was liquid, and added a little freshly cut sodium (another fascinating material - a metal you can cut with a knife) and the solution went blue with free electrons and for a while I just looked at it, fascinated at the thought that I could, in a way, 'see' electrons.

Anyway, I am off now to do another experiment in the book - on page 62 it tells you that if you mix cornflour with cooking oil you can make it creep towards a balloon...but first I am going to experiment with another material that can't make up its mind. On the table it is solid but inside my mouth it turns into the most darkly-delicious liquid that has ever been invented.

Happy Easter!