July 12, 2012

Exploring Non-Newtonian Fluids Using Corn Flour

Brett Smith for - Your Universe Online

If you´ve ever run across a section of beach where a wave has just receded, you know that the wet sand there can act and feel as hard as concrete. The thick mixture of sand and water that can be poured like a liquid but is hard when struck is an example of what scientists call a non-Newtonian fluid.

According to a study published today in Nature, researchers from the University of Chicago have demonstrated that a compression of particles within the fluid and around a strike zone will create a hard, resistant surface.

Using another non-Newtonian fluid, a mixture of water and corn starch, study authors Scott Waitukaitis and Heinrich Jaeger set up an experiment hoping to identify the mechanism that gives these fluids their signature characteristic.

The two physics professors equipped an aluminum rod with an accelerometer and used "tracer particles" within the solution to take slow-motion X-ray images in the middle of the bowl. They also used a laser line across the surface of the bowl. As the rod struck the surface of the mixture, the sensors measured where the forces were distributed at the bottom of the bowl.

The experiment allowed the team to capture the fluid´s particles at work as they crashed together and formed a resistant surface.

"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," lead author Waitukaitis told the BBC News.

"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."

"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,” he added.

The result of these two behaviors is the appearance of the striking object sinking slightly into the fluid as the particles coalesce around the contact surface. The fluid´s contact surface eventually creates an increased resistance to the striking object.

Waitukaitis also said that while physicists have had a sense of how non-Newtonian fluids behave, this mechanism has never been articulated before.

"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."

The practical implications for this new study could be wide ranging. Non-Newtonian fluids could be used in construction to develop new kinds of cements or foundational materials.

The latest research could also be used to save lives in the form of more robust armored vests that conform to a person's torso in a fluid manner, but gets as hard as it becomes struck by bullets or weapons.