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Monday, July 16, 2012

Corn flour mixture's strange properties explored

12 July 2012

Similar effects are at work in the dry area around the
weight-bearing foot on wet sand

Scientists are finally getting to the bottom of the mechanism behind the familiar "kitchen chemistry" experiment using water and corn flour.

A thick mixture of the two pours like a liquid but is hard when struck; the same idea is used in some body armour.

But exactly what is going on within these mixtures has remained a mystery.

Now researchers reporting in Nature have shown how compression of the particles just below the strike area jam together when under a force.

The corn flour (corn starch in the US) and water mixture is just one example of what are known as non-Newtonian fluids, whose viscosities (resistance to flow) behave differently from the more familiar, "Newtonian" fluids from everyday life.

For the case of corn flour - or quicksand, or the familiar example of the wet sand just ahead of a receding wave - the behaviour arises because of interactions between particles that are just millionths of a metre across. (...)

These mixtures will pour or drip, but when exposed to fast movements, they seem to get radically thicker - leading many to attempt the "trick" of walking on pools of corn flour mixture - or even make "corn flour monsters" using a speaker cone.

"The corn starch grains are like tiny little rocks bobbing around in the water, very densely packed but not so densely that they're touching each other," explained lead author of the study Scott Waitukaitis of the University of Chicago in the US.

Just what is going on when such mixtures are struck by a foot or scooped quickly with a spoon has been explained away as a "solidification" process, but Mr Waitukaitis told BBC News that "this hasn't ever been articulated very well".

"If you asked them, a lot of people - even in our field - would have said that if you hit corn flour and water you're just transmitting stress to the bottom of the container via some solid-like object - but that doesn't answer how this object forms." (...)

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