Splashing, spreading, sticking leads to CAREER award for MechSE’s Randy Ewoldt


As children, in science class we learn how to distinguish between simple solids, liquids, and gases. We understand, for instance, that a fluid such as water flows when any force is applied. But when a material can evolve from one to another—say, from a liquid to a solid and back again due to applied force—it becomes more difficult to understand its properties and to learn to control it enough to create useful applications for it.

Assistant professor Randy Ewoldt
Assistant professor Randy Ewoldt
This concept of a solid becoming a liquid when stirred or shaken and then turning back into a solid when the agitation ceases involves time scales. But once you stop stirring, it might still look like a liquid before enough time passes for the material to “re-heal” itself back into a solid. In other words, the time scale isn’t zero and the change doesn’t happen instantaneously.

The specific time scale aspect of certain thixotropic materials is what MechSE assistant professor Randy Ewoldt is focused on. Ewoldt was honored with the prestigious NSF Faculty Early Career Development (CAREER) Program award, a grant for him to continue this research (titled “Thixotropic Yield Stress Fluids – Splashing, Spreading, Sticking”) over the next five years, through April 2019.

Ewoldt’s research is unique in that thixotropic, time-dependent yield stress fluids are still not completely understood in the same way that polymer melts and solutions are.

“The scientific community has thought about this in a lot of other ways, but not in these sorts of ways. Our proposal is to do a very thorough experimental study of model materials that have different time scales of this solid-to-liquid transition. We’ll look at droplets splattering against a surface, and experiment with jet impacts on a surface to see what happens with things like coating, run-off, and splash dynamics,” he said.

Ewoldt identified several broad societal impacts that could come from his efforts to understand the behavior of fluids that are rheologically complex, including designing new fluid materials to fight fires and developing novel manufacturing coating technologies. However, his research is intended to be of more permanence and more fundamental than any particular application.

“The idea is that you can take a thixotropic gel and send it through a pump like it’s a fluid, spray it out the end of a fire hose, and when it hits the wall it can stick and stay, or splatter, or a combination of both, depending on the thixotropic time scale. If it was water, you’d get splatter back, it’s going to flow down, and you’ll have a thin layer that just keeps getting thinner and thinner as time goes on. But if you can flow a material that looks like a fluid under flow conditions, and re-solidifies at rest, then what you can do with this is blanket a gel on a wall to coat it, which doesn’t slough down and cause water damage below, and it could splat and stick in place,” said Ewoldt.

Although his research will be focused within his research group and from his lab here at Illinois, a Minnesota-based company called Earth Clean—which makes a product that is added to water as a fire suppressant—will serve as one translational research partner to help determine which experimental conditions are relevant for their particular application in the real world. “They view this research as very fundamental and relevant to their business. But we’ll be exploring a wider space than just that one application,” said Ewoldt.

CAREER awards are granted based on outreach and educational efforts alongside the research. Ewoldt will continue to develop his “Rheology Zoo” collection of learning modules to be even bigger and to eventually disseminate it much more broadly beyond the university, as well as to high school students and college students outside of engineering. “I’ll also continue my collaboration with the School of Art and Design, in particular with professor Cliff Shin, giving guest lectures and co-teaching to some extent, to expose junior-level industrial design students to rheology as a disruptive technology, and to push graduate engineering students to think creatively about rheology, applied to product design.”