Money Matters - Simplified

Strange characteristics of corn flour mixture's unravelled

According to a new study, it is due to the compression of particles just below the strike area which jams together when under a force, that the thick mixture of water and corn flour which otherwise pours like a liquid, solidifies.

The study
The researchers had set up a hi-tech version of the familiar kitchen experiment. They used an aluminium rod with an accelerometer and fired a laser line across the surface of a bowl of corn flour mixture.

Everything was captured on a slow-motion camera, 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 corn flour and water mixture is just one example of what are known as non-Newtonian fluids, whose viscosities behave differently from the more familiar, "Newtonian" fluids from everyday life.

"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.

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. But according to Waitukaitis the "solidification" process, hasn't ever been articulated very well.

The study findings were published in journal Nature.