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Dancing Cornflour – Polymer properties and Sound Waves
Cornflour (1 or 2 packs), water, bowl, speaker (optional), cling film (optional), signal generator/ipod (optional), food colouring (optional)
Place the cornflour in a bowl and slowly add small amounts of water. DO NOT ADD TOO MUCH WATER. Once all the cornflour has been dampened, it will form a sloppy paste, like that seen above that will be resistant to the touch. This can be picked up and rolled around on you hands like a solid until your movement slows or stops whereupon it will resort to a liquid state again. Hitting it will cause it to act like a solid – similarly if you bounce something off its surface, but moving slowly you should be able to run your fingers through it. There are lots of strange effects this has and you will discover many of them simply by having a play! Also try rolling a ball of it and throwing it at something solid. During its flight it is a liquid but when it hits something it turns solid once again.
If you have the speaker lay it on its back so the speaker faces up and cover the surface with cling film. Place a small amount of the cornflour water mix into the speaker cone. Then play music or use a signal generator to produce a 50-60 Hz sine wave. The material will respond to the sound waves by moving and growing, becoming hard in places and soft in others and this provides and excellent visual simulation of sound waves. Introducing a drop of food colouring allows you to track the net movement of molecules through the fluid due to the sound waves.
The cornflour water mix is a well known example of a non-Newtonian fluid. Simply, a Newtonian fluid is one moves faster the more force you place on it, like water flowing through a hose – the harder you push it, the faster the water flows. Non-Newtonian fluids are fluids that, for various reasons ,do not obey this simple everyday logic that we are used to. This one in particular is due to the polymer structure of the cornflour – which is a long chain glucose polymer (starch), that acts like microscopic spaghetti. When you touch the fluid slowly, the long strands slide past each other easily, but when you interact with the fluid quickly the strands tangle (try stirring spaghetti quickly once it’s cooked) and the fluid changes into a solid until the force dissipates.
When on a speaker, the sound waves provide the forces to solidify the fluid – but only at certain places due to the wave nature of sound. Constructive interference causes solidification and can cause the fluid to erupt into tentacle shapes, whereas destructive interference leaves the fluid in its liquid state. A strong enough speaker can bounce the entire mass of cornflour off its cone, but as soon as it leaves the surface it becomes a liquid again.