Study Explains Complex Motion of Soft Materials

Explaining how toothpaste remains in its tube without being exuded when the cap is opened and the reason behind abrupt loosening of solid ground into a landslide has been challenging.

A new study from engineers at the University of Illinois, Urbana-Champaign uses simple experiments to explain how a better understanding of flowing motion of soft materials will help design new materials and could help predict some natural disasters. Image Credit: U.S. Geology Survey.

For decades, researchers have been unable to define how exactly soft materials flow and seize. However, a new study describes this complex motion with the help of comparatively easy experiments.

The potential to describe—and ultimately estimate—the flow of soft materials will be advantageous to people handling everything from spreadable cheese to avalanches.

Conducted at the University of Illinois at Urbana-Champaign, the study has been published in the Proceedings of the National Academy of Science.

We are finding that soft material flow is more of a gradual transition rather than the abrupt change the current models suggest.

Simon Rogers, Chemical and Biomolecular Engineering Professor, University of Illinois at Urbana-Champaign

Rogers, who headed the study, is associated with the Beckman Institute for Advanced Science and Technology at the University of Illinois at Urbana-Champaign.

A new testing protocol designed by the research team enables scientists to quantify the individual liquid-like and solid-like behaviors of such materials separately. This has never been achieved before, stated Gavin Donley, a graduate student and lead author of the study.

In the laboratory, the research group subjected a range of soft materials—a xanthan gum, a polymer microgel, a glass-like material, and a filled polymer solution—to shear stress and quantified the individual solid-like and liquid-like strain responses with the help of a device known as a rheometer.

Our experiments show us a much more detailed and nuanced view of soft material flow. 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.

Gavin Donley, Graduate Student, University of Illinois at Urbana-Champaign

According to Rogers, the instant aim of the researchers is to translate the experimental observation into a theoretical model that can estimate soft material motion.

The existing models are insufficient to describe the phenomena that we have observed. Our new experiments are more time-consuming, but they give us remarkable clarity and understanding of the process.

Simon Rogers, Chemical and Biomolecular Engineering Professor, University of Illinois at Urbana-Champaign

Rogers added, "This will allows 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.”

The study was financially supported by the National Science Foundation, U.S. Department of Energy, Sandia National Lab, and the Anton Paar VIP program.

Journal Reference

Donley, G. J., et al. (2020) Elucidating the G″ overshoot in soft materials with a yield transition via a time-resolved experimental strain decomposition. Proceedings of the National Academy of Science.


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