Amazing Maizena (Cornflour)
A thick white paste can be picked up and rolled around in the hands as long as the ball is kept moving. As soon as the ball is left alone the paste runs through the fingers.
- Bowl and spoon to mix
Mix water and cornflour to make a thick paste. The paste should be thick enough that it is (very) hard to move a spoon through. It should also appear to crumble when taken out of the mixing bowl.
Cornflour (known as Maizena in some countries and corn starch in others) has a very strange property when mixed with water. Made to the right consistency it will flow when left alone, but shatter when hit with a hammer.
What we have here is an example of a non-Newtonian fluid. It does not obey the usual rules of fuilds such as air or water. The cornflour paste is a shear-thickening (or dilatant) fluid – it becomes thicker, or more viscous, when it tries to move quickly. Tomato ketchup on the other hand is an example of a shear-thinning (or thixotropic) fluid.
Cornflour (Maizena) is made up of many small grains of starch. These are able to lock together when they are forced to move suddenly, however, left on their own the water is enough to allow them to move past one another.
A search for Non-Newtonian fluid on YouTube will show this paste (which is also known as “oobleck” after a character in a Dr Seuss book) dancing on loudspeakers and people running over tubs full of the mixture, only to sink into it when they stop moving.
If hit with a hammer so that it does indeed shatter the flying droplets may pose a hazard. Equally drops of this mixture on hard floors can be very slippery.