Understanding how soft materials fail under stress is critical to solving engineering challenges as disparate as pharmaceutical technology and landslide prevention. A new study linking a spectrum of soft material behaviors, previously thought to be unrelated, led researchers to identify a new parameter they call the brittleness factor, which allows them to simplify the failure behavior of soft materials. This will ultimately help engineers design better materials to meet future challenges.
Simon Rogers, a professor of chemical and biomolecular engineering at the University of Illinois at Urbana-Champaign, and Krutarth Kamani, a graduate student, specialize in determining how soft materials yield under stress and have demonstrated how solid and liquid physical states can coexist in the same material. This area is of great interest because of its importance to industrial, environmental and biomedical applications.
Along the way, the team identified a communication breakdown between scientists working in this area, causing a bottleneck between theoretical understanding of soft materials behavior and real-world applications.
When soft materials (natural or synthetic) deform under pressure, they reach a critical point where they either return to their original shape or undergo a permanent deformation, like stretching or a piece of elastic breaking. This process is known as creep. A gradual creep transition is called ductile behavior, while an abrupt one is called brittle behavior, the researchers said.
“At a recent conference, we realized that all of us studying soft materials from across Europe and North America couldn’t agree on what the connection is between brittle and ductile behavior and how to define it.”
In the study, published in the journal Proceedings of the National Academy of Sciences, Rather than considering the behavior of soft materials as one or the other (brittle or ductile), Rogers’ team considers a spectrum of creep behaviors. This allowed the team to build a continuous model, which led them to discover the brittleness factor. This factor is critical to determining how and why soft materials fail.
Essentially, brittleness affects how a material permanently deforms under stress. The team’s model indicates that the higher the brittleness factor, the less a soft material will permanently deform before yielding.
As in the team’s previous studies, the model was developed and tested using data from numerous experiments that subjected various soft materials to stress while measuring individual deformation responses using a device called a rheometer.
“We didn’t expect this study to explain as much as it does,” said Rogers, who is also an affiliate of the Beckman Institute for Advanced Science and Technology at the University of Illinois. “What we got was a way to bring a bunch of soft material behaviors together under the same physics umbrella. Previously, they had been studied independently or maybe all applied simultaneously, but they were never thought to be physically or mathematically connected.”
This discovery will allow researchers to explain precisely why some materials are more resistant to rapid deformation than others, a question that has eluded researchers for decades.
“This single parameter surprisingly connects many puzzling observations that researchers have found over the years,” Kamani said.
“This work marks the point where we are getting closer to the top of the hill in understanding the behavior of soft materials,” Rogers said. “We’ve always felt like each step takes us higher, but with no end in sight. Now we can see the top of the hill, and we’re closer to the top and free to move forward in any direction we want.”
The National Science Foundation supported this research.