Physics of the thixotropic solution

Corn flour and water – it’s a non-Newtonian fluid - but exactly how does it work? This is a question that physicists from the University of Chicago have attempted to answer.

When corn flour and water are mixed, they form a thixotropic solution – a liquid that spontaneously forms a solid when hit or thrown from hand to hand. But the physics behind this transformation remained unclear.

In a first-of-a-kind experiment, Scott Waitukaitis and Heinrich Jaeger studied direct compression of these suspensions – liquids laden with micron-sized particles. They used a combination of high- and low-tech instruments – including force sensors, laser sheets, X-rays, high-speed cameras taking images at 10,000 frames a second and an industrial cement mixer – to see what happened when a rod was driven into the corn flour and water mixture.

“We found that when you hit the suspension, a solid-like column grows below the impact site,” said Waitukaitis, a physics graduate student. “The way it grows is similar to how a snowplow works. If I push a shovel in loose snow, a big pile of compacted snow grown out in front of the shovel, which makes it harder and harder for me to push.”

[caption id="attachment_29220" align="alignright" width="200" caption="Researchers struck a corn flour water mix with a rod to understand what happens in a thixotropic solution"][/caption]

In the suspension, individual corn flour grains pile up in front of the impacting object and become temporarily jammed after compression has halted movement. The researchers found that their impacting rod initiated a shock-like, moving front that starts directly beneath the impacting object and then grows downward, transforming the initially liquid suspension into a temporarily jammed state.

“It essentially grows its own solid as it propagates,” said Jaeger, Professor of Physics.

The scientists – who published their work in Nature – have called this process “impact-activated solidification”, and are extending their work by teaming up with researchers at Leiden University to model the propagating shock fronts in more detail.

Non-Newtonian suspensions have the potential to be used as the basis for new body armour – as well as in construction and biomedicine.

“It would be liquid, so it would conform to a particular shape, and when it gets hit hard, it knows it needs to become hard,” said Waitukaitis. It’s a smart material that increases resistance with the amount of force applied against it.

• Impact-activated solidification of dense suspensions via dynamic jamming fronts