An Engineer in Wonderland – Cornflour heaven

Along with a visitor last night came her son.


And along with the son came a jar of gunk which reminded me of the cornflour and water mix I played with years ago.

And what pleasure, it was cornflour and water.

For the uninitiated, this stuff behaves in a very odd way and will keep kids and the odd engineer entertained for hours.

When poured and stirred slowly, it is a liquid.

When thumped, it momentarily becomes a solid.

If you hit is hard enough, it will actually shatter before turning back into drops of liquid.

In between these extremes, you can quickly scoop up a lump and hold it, providing you keep manipulating it. As soon as you stop, it runs away between your fingers.

I was once told this is thixotropic behaviour and I have believed this for years. I assumed thixotropic meant changes viscosity when manipulated – non-drip paint is thixotropic.
But I asked Mr Google this morning and it turns out non-drip paint is indeed thixotropic, but cornflour is not.

So far, I am most convinced by Wikipedia’s description:

“An incorrect example often used to demonstrate rheopecty is cornstarch mixed with water…..However, cornstarch in water is actually a dilatant fluid  since it does not show the time-dependent, shear-induced change required in order to be labelled rheopectic. These terms are often and easily confused since the terms are rarely used; a true rheopectic fluid would when shaken be liquid at first, becoming thicker as shaking continued.”

So cornflour and water is

a, dilatant
b, fun


Respond below, or to



  1. This rather wonderful video has been bought to my attention.

  2. Saw an idea on Half bakery for custard filled speedbumps that could use this principle. Soft if you drive slowly, hard if you drive fast.
    Details here:

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