11/9/2026
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A paper led by mechanical science and engineering researchers M. Tanver Hossain, postdoctoral associate, and professor Randy H. Ewoldt has received the Walters Prize for the best paper published in the Journal of Non-Newtonian Fluid Mechanics (JNNFM). The prize was presented October 27 at the Society of Rheology Annual Meeting in Boston.
The Walters Prize is awarded annually to the best paper among those published in the journal during the previous year, as judged by the prize committee. All full-length research papers published in the award year are eligible. The prize is named for Professor Ken Walters FRS, the journal’s founding editor.
The winning paper, “The critical plastocapillary number for a Newtonian liquid filament embedded into a viscoplastic fluid,” asks a practical question: how small can embedded 3D printing go?
In embedded 3D printing, a nozzle deposits a liquid ink inside a supporting bath of a “yield-stress” material, a soft solid that behaves like a gel at rest but flows when pushed hard enough. The bath holds the printed filament in place. But surface tension constantly tries to break a thin liquid thread into droplets, the same effect that turns a stream from a faucet into drips. The narrower the filament, the stronger this pull becomes relative to the bath’s ability to resist it, so there is a minimum diameter below which printed lines break apart.
A fundamental discovery is at the heart of the paper: the physical law that sets this limit. The prevailing hypothesis held that the limit is reached when the filament diameter equals a length scale set by the yield stress and surface tension, an explanation that only worked if the liquid’s surface tension was assumed to be greatly reduced.
The team proved this hypothesis false. They showed instead that stability is governed by a dimensionless quantity called the plastocapillary number, which compares the strength of the yield-stress bath to the surface tension, and that its critical value is not one, as the earlier picture implied, but about 0.21. Experiments across a wide range of materials and conditions consistently landed on that value, establishing it as a basic quantity that anyone working with these materials can now use to predict the smallest stable filament.
“It’s a rare thing for a student to discover the critical value of a dimensionless group. That’s foundational knowledge, the kind that any fluid dynamics problem is built on, and Tanver found it,” said Ewoldt. “It’s a cousin of other classic thresholds in yield-stress fluids, like whether a sphere or bubble stays suspended or sinks.”
From a fundamental limit to beating it
The work was part of a DARPA-supported collaboration led by Douglas Fudge, professor of biological sciences at Chapman University and a world expert in hagfish biology, with co-principal investigators Ewoldt, whose expertise is in rheology and fluid mechanics, along with MechSE professor Sameh Tawfick, whose expertise is in manufacturing. The team’s goal was inspired by the hagfish, which defends itself by releasing a slime reinforced with remarkably strong, fine threads.
Knowing exactly where the limit sat told the team what they had to overcome. In follow-up work published in Nature Communications, they invented a new manufacturing technique, embedded solvent exchange, that beats the limit and rapidly prints fine, continuous, soft fibers.
“Understanding the fundamental limit was what let us design a way around it,” said Ewoldt. “That’s the payoff of combining fluid mechanics, manufacturing, and biology on one team.”
That advance in turn led to an engineering first. In a paper in Advanced Science, the team created the world’s first synthetic versions of hagfish slime threads and laid out design principles for deployable fibers inspired by hagfish defense.
“This discovery grew out of many discussions, shared ideas, and different perspectives. I am grateful to have been part of a team where we could learn from one another and turn those ideas into something meaningful,” said Hossain.
The JNNFM paper is part of Hossain’s PhD thesis, which received the department’s 2026 Stanley I. Weiss Outstanding Thesis Award in mechanical engineering. Hossain is now a postdoctoral researcher in MechSE.
The author team spans career stages and institutions. Wonsik Eom was a MechSE postdoc at the time of publication and is now a professor at Dankook University in South Korea. Arjun Shah contributed as a MechSE undergraduate and is now a PhD student at Stanford University. Andrew Lowe is a postdoctoral researcher in biological sciences at Chapman University.
The work was supported by the Defense Advanced Research Projects Agency (DARPA).
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Randy Ewoldt is the Alexander Rankin Professor in mechanical science and engineering in The Grainger College of Engineering.
Sameh Tawfick is a professor and Ralph A. Andersen Faculty Scholar of mechanical science and engineering in The Grainger College of Engineering.