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
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