How soft materials flow?

New approach to explaining soft material flow may yield way to new materials, disaster prediction.

Materials that display yielding behavior are utilized in numerous applications, from spreadable foods and cosmetics to direct-write three-dimensional printing inks and filled rubbers. Their key design feature is the capacity to progress typically from solid to liquid under sufficient load or deformation.

Despite the ubiquity of materials that exhibit yielding, an understanding of the transition has remained elusive due to experimental limitations of the simple rheological tests typically used to study it.

A new study by the University of Illinois explains this complex motion using relatively simple experiments.

The ability to characterize – and eventually anticipate – soft material flow will benefit people dealing with everything from spreadable cheese to avalanches.

Chemical and biomolecular engineering professor Simon Rogers, said, “We are finding that soft material flow is more of a gradual transition than the abrupt change the current models suggest.”

Scientists developed a new testing tool to determine individual solidlike and liquidlike behaviors of these materials separately. In the lab, scientists subjected different soft materials including a polymer microgel, xanthan gum, a glass-like material, and a filled polymer solution. A rheometer device was used to shear stress and measured the individual solidlike and liquidlike strain responses.

Gavin Donley, a graduate student and lead author of the study, said, “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.”

Scientists want to turn this experimental observation into a theoretical model that predicts soft material motion.

Rogers said“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. 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 National Science Foundation, U.S. Department of Energy, Sandia National Lab, and the Anton Paar VIP program supported this research.

Journal Reference:
  1. Gavin J. Donley et al. Elucidating the ‘G’ overshoot in soft materials with a yield transition via a time-resolved experimental strain decomposition. DOI: 10.1073/pnas.2003869117

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