Simon Rogers (left) with graduate student Gavin Donley (right). Photo: L. Brian Stauffer.
Simon Rogers (left) with graduate student Gavin Donley (right). Photo: L. Brian Stauffer.

How does toothpaste stay in its tube and not ooze out when the cap is removed? What causes seemingly solid ground to suddenly break free into a landslide? Defining exactly how soft materials flow and seize has eluded researchers for years, but a new study has explained this complex motion using relatively simple experiments. The ability to define – and eventually predict – soft material flow will benefit people dealing with everything from spreadable cheese to avalanches.

The study was conducted by researchers at the University of Illinois at Urbana-Champaign, who report their findings in a paper in the Proceedings of the National Academy of Sciences.

"We are finding that soft material flow is more of a gradual transition rather than the abrupt change the current models suggest," said chemical and biomolecular engineering professor Simon Rogers, who led the study and is an affiliate of the Beckman Institute for Advanced Science and Technology at the University of Illinois.

Rogers and his colleagues developed a new testing protocol for measuring the individual solid-like and liquid-like behaviors of soft materials separately. This had never done before, said Gavin Donley, a graduate student and lead author of the paper.

In the lab, the team subjected a variety of different soft materials – a polymer microgel, xanthan gum, a glass-like material and a filled polymer solution – to shear stress, and then measured the individual solid-like and liquid-like strain responses using a device called a rheometer.

"Our experiments show us a much more detailed and nuanced view of soft material flow," Donley said. "We see a continuous transition between the solid and liquid states, which tells us that the traditional models that describe an abrupt change in behavior are oversimplified. Instead, we see two distinct behaviors that reflect energy dissipation via solid and fluid mechanisms."

The team's immediate goal is to turn this experimental observation into a theoretical model that predicts soft material motion.

"The existing models are insufficient to describe the phenomena that we have observed," Rogers said. "Our new experiments are more time-consuming, but they give us remarkable clarity and understanding of the process. This will allow us to push soft materials research forward in a slightly different direction than before. It could help predict the behaviors of novel materials, of course, but also help with civil engineering challenges like mudslides, dam breaks and avalanches."

This story is adapted from material from the University of Illinois at Urbana-Champaign, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.