London: Ever wondered why a thick mixture of water and corn flour pours like a liquid but hardens when struck? It`s because the compression of the particles just below the strike area jam together when under a force, a new study has found.
The corn flour and water mixture is just one example of what are known as non-Newtonian fluids, whose viscosities or resistance to flow behave differently from the more familiar, "Newtonian" fluids from everyday life.
For the case of corn flour -- or quicksand, or 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," said study author 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 Waitukaitis told BBC News that "this hasn`t ever been articulated very well".
To find out, the researchers 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 team 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 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.
The researchers, who detailed their findings in journal Nature, said 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, 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."