When you mix cornstarch and water, weird things happen.
Swish it gently in a bowl, and the mixture
moves like a liquid.
Squeeze it, and it starts to feel like paste.
Roll it between your hands, and it
solidifies into a rubbery ball, until you
try to hold that ball in the palm of your hand,
and it loses its structure and dribbles away.
To many who have played with this material,
perhaps as children, its strange behavior is nothing new.
But understanding exactly how why and when
this material will act a certain way
has always been rather unpredictable.
But now, a team of MIT engineers have
developed a mathematical model that can accurately
predict this material's behavior under various conditions.
A single particle of cornstarch is about 1 to 10 microns wide,
and about 100 times smaller than a grain of sand.
It turns out that particles at such a small scale
experience effects that larger particles do not.
Because cornstarch particles are so small,
they can be influenced by temperature
and by electric charges that build up
between them, which causes them to slightly repel
against each other.
So as long as you move slowly, the grains
will repel and slide past each other like a fluid.
But if you do anything too fast, you'll overcome that repulsion.
The particles will touch, there will be friction,
and it will act as a solid.
The researchers incorporated equations into their model
to describe the effects of particle repulsion
and the speed at which the material is deformed to predict
whether it would behave as a solid or a liquid
under various scenarios.
The researchers say the new model
can be used to explore how various ultra-fine particle
solutions, such as cornstarch and water,
behave when put to use as, for instance,
fillings for potholes or bulletproof vests.