Corn flour mixture's strange properties explored
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 thousandths of a millimetre 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."
To find out, Mr Waitukaitis and Prof Heinrich Jaeger set up a hi-tech version of the familiar kitchen experiment, equipping an aluminium rod with an accelerometer and firing a laser line across the surface of a bowl of corn flour mixture.
A slow-motion camera captured what happened as the rod struck the surface of the mixture, and sensors measured where the forces were distributed at the bottom of the bowl.
The pair also used "tracer particles" within the mixture to take slow-motion X-ray images of what was going on in the middle of the bowl, finding that two effects were at work in the process.
"The simplest way to look at it is that if I hit the surface really hard, I cause a build-up of grains in front, kind of like a snow plough," Mr Waitukaitis explained.
"If you push a shovel through loose snow it gets harder and harder as you go, because you're getting more and more snow building up in front of you - the solidification is kind of a snow plough of these grains smashing into each other."
This solid region has the same "footprint" as the impacting object, and extends to a depth as much as 20 times as wide as the object itself - but not necessarily all the way to the edge of the container.
"The second thing that happens is that as you create this snow plough, it's being pushed through this surrounding fluid. I can't push one solid region of the fluid without pulling on the surrounding regions."
The slow-motion video showed how a depression surrounding the rod grew with time, drawing some of the mixture down and appearing to sink into the surface.
A similar effect can be seen when walking on a beach where a wave has just receded; the weight-bearing foot is surrounded by a dry-looking area as the water around the impact region is drawn downward.
Understanding just what is going on in these systems has wider implications beyond the kitchen, Mr Waitukaitis explained.
"It's a lot more than just running across pools for game shows in Japan," he said. "The big picture is that this is a kind of material that knows how to change its properties, which is very powerful.
"You can think of industries ranging from construction, working with new kinds of cements, to using them as an addition to things like bullet-proof vests that conform to a person's shape but gets as hard as it needs to be depending on how hard it's hit."