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Graphic representation of a soft solid material.
The new approach to the characterization of soft materials published by Drs. Chandler Benjamin and Alan Freed enables researchers to measure their normal stress difference. | Image: Texas A&M Engineering

Toothpaste and Jell-O may not be the first things that come to mind when it comes to engineering, but they — and materials like them — are the beneficiary of ongoing research to better measure normal stress differences in soft materials. 

Drs. Chandler Benjamin and Alan Freed from the J. Mike Walker '66 Department of Mechanical Engineering at Texas A&M University, recently published their research in the journal Physics of Fluid, detailing a new approach to measuring soft materials that were previously unable to be characterized.

"This approach opens the door to the measurement of normal stress differences on soft solid materials — something along the lines of rubber — that would not have been possible previously," said Benjamin, assistant professor in the mechanical engineering department. "This paper shows that there is a way to measure normal stress differences on soft solid materials within a single experiment. This would not be possible with the current theoretical underpinnings of existing experimental methods."

Normal stress difference in soft materials is a naturally occurring phenomenon Benjamin said is often taken for granted. While not very common, a few well-known examples are die swell — which occurs in polymer processing when the material is passed through a die — and the Weissenberg Effect — a phenomenon in which a solution of elastic liquid will rise up when a spinning rod is inserted into it.

To measure these soft materials, researchers currently need to conduct two separate experiments — each with two interpretations — in order to measure the normal stress difference of a material. 

"This new approach requires one experiment and one interpretation," said Freed, professor in the mechanical engineering department. "It opens up the possibility of measuring normal stress differences for a wide range of materials that previously would not have been possible to do."

The researchers said the measurement of these normal stress differences is key to the characterization of all viscoelastic fluids and solids. Utilizing the new interpretation outlined in their paper, researchers will have the ability to characterize soft viscoelastic solid materials more efficiently, allowing for the enhanced design of common materials — like toothpaste — used in daily life.

As the research advances, the next steps will be to apply this new approach to various soft materials to measure their normal stress differences. Although this research does not currently have direct practical applications for the average person, Benjamin said he hopes this scientific advancement will open the door to further research and uses in the future.

“I’m hoping this will spur further research into soft solid materials characterization,” Benjamin said. “Right now, people just study viscoelastic fluids, so hopefully this generates interest, a better understanding and better method of characterizing of soft materials.”