Cornflour Isotropy

Liquid or Solid?

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Purpose -

A great way to help students see that the line between solids and liquids is not always clear. Also shows the effect of large molecules on a substance.

Nature - 

Demonstration

Materials - 

Cornflour, water, bowl 

Method - 

· Mix 1½ cups of cornflour with 1 cup of water in the bowl

· Mix in well until a slow running mixture is obtains.  It's runny like a liquid, right!?

· Hit or slap the mixture hard with your hand and you'll find it acts more like a solid.

· Try kneading the mixture very quickly and you may even be able to pick it up as an almost solid lump.

Safety -

Nil

Explanation -

Cornflour particles are very large and complex.  When given time to move they can flow around each other, but if forced to move quickly the become tangled and can't move fast enough to flow.

Notes -

The proper name for a liquid with these properties is a "Shear Thickened Liquid" or a "Non-Newtonian Liquid" because it does not follow the logical laws set out in Newtonian physics.  The process of a liquid acting as a solid temporarily is described as isotropy.  This mixture does not keep for more than a couple of days.