By Amber Rose
Sitting in a restaurant, you reach for the ketchup bottle, eyeing the basket of fries in front of you. You give the bottle a shake, then a tap. For a moment, nothing happens — the ketchup clings stubbornly to the glass. Then, all at once, it lets go and rushes out, sometimes in a steady stream, sometimes in a messy surge that threatens to flood the basket.
That awkward moment when ketchup stops behaving like a solid and suddenly starts flowing like a liquid is called “yielding.” Scientists see the same kind of behavior in many everyday and advanced materials, from toothpaste, paints and concrete to 3D-printing inks and electrodes used in next-generation batteries. Yet, what actually causes a material to hold its shape one moment and suddenly let go the next has been surprisingly hard to pin down, especially deep inside dense, opaque fluids where particle motion is difficult to see.
In a new study from researchers at the U.S. Department of Energy’s (DOE) Argonne National Laboratory and the University of Chicago, scientists used powerful X-ray beams and sophisticated computing resources to track “ketchup-like” materials as they yielded and flowed. They found that tiny differences in how particles attract or repel each other can make a material flow smoothly, flow in uneven bands, or even stop flowing and turn solid again while under stress. The results could help engineers design better consumer products and more reliable manufacturing processes by precisely controlling when and how soft materials begin to flow.
“Yielding is the transition from solid-like behavior to liquid-like behavior,” explained Argonne Assistant Physicist Hongrui He. “By applying a force or stress, we are able to manipulate the state of matter. There is no perfectly solid or perfectly liquid material — everything is somewhere in between, and yielding is the shift from one to the other. Given enough time, even a mountain can behave like a very slow-moving fluid.”
To study this transition, the team created two closely related materials, both made of tiny particles suspended in liquid. In one, the particles were prepared so they mostly repelled each other. In the other, the researchers added a salt solution that subtly altered the particles, so they were weakly attracted and tended to stick together.
At Argonne’s Center for Nanoscale Materials, a DOE Office of Science user facility, the size, composition and surface charge of the samples were carefully characterized to ensure that any changes in flow behavior came from particle interactions rather than changes in the particles themselves.
When the samples were not under stress, they looked almost identical. The striking differences only appeared when the researchers applied force and watched how each material flowed.
“When the particles repel each other, the material changes shape in a very even way,” He said. “It flows in a predictable way, without forming large weak spots inside.”
The picture changed when the particles were made slightly attractive. In this case, the particles tended to clump together into dense regions, leaving behind pockets of empty space. Under stress, some parts of the material started to move while neighboring parts stayed stuck. The material split into “shear bands” — regions that flowed at different speeds.
“In the attractive system, parts of the material are almost frozen while other parts are flowing,” said Wei Chen, a chemist from Argonne and a CASE scientist at the University of Chicago Pritzker School of Molecular Engineering (UChicago PME). “That leads to more complex behavior, such as delayed yielding and resolidification, which you do not see in simple fluids.”
Delayed yielding occurs when a material resists flow for a while after a stress is applied and then suddenly begins to move. Resolidification is the opposite: the material flows for some time and then abruptly stops and behaves like a solid again, even though the applied stress has not changed. These effects help determine whether a material spreads smoothly in use or instead suddenly stiffens, leading to problems such as clogs in industrial processes.
To uncover what was happening inside the materials, the researchers combined standard rheology — measurements of how a material flows and changes under stress — with a technique called X-ray photon correlation spectroscopy (XPCS) at beamline 8-ID at the Advanced Photon Source, a DOE Office of Science user facility at Argonne. While rheology measurements revealed how the whole sample responded, XPCS, which uses a very bright X-ray beam, allowed the team to track tiny fluctuations in scattered X-ray signals that revealed how groups of particles move over time.
“The unique aspect of our approach is that we can measure the motion of the small particles and the overall material response at the same time,” Chen said. “That allows us to directly connect microscopic dynamics to macroscopic behavior in real time.”
Even with these tools, experiments alone cannot capture every detail of the particle motion. To fill in the picture, the team used computer simulations to model dense suspensions of many interacting particles under flow, making it possible to track the motion of individual particles. Simulations were performed on Bebop, a high performance computing cluster at Argonne’s Laboratory Computing Resource Center.
“In experiments, the material is dense and opaque, so you can’t track every single particle,” said Heyi Liang, a research associate at Argonne and postdoctoral scholar at the University of Chicago. “With simulation, you can. We built the simplest model that still captures the most important parts, including delayed yielding and resolidification. We then used it to understand what is happening at the boundaries between flowing and non‑flowing regions.”
The simulations showed that weak junctions between shear bands — areas where particles are less well connected and have more room to move — play a key role. Under small stresses, these junctions hold, and the material creeps slowly. As stress continues, some junctions suddenly fail, allowing bands of particles to slip past each other, producing delayed yielding. As the system continues to evolve, new junctions form and lock the structure again, leading to resolidification.
By tying these microscopic events to measurable quantities from XPCS and rheology experiments, the team built a consistent picture that matched both experiment and simulation.
“Our findings bridge the microscopic and macroscopic worlds of soft matter,” said one of the study’s coauthors, Juan de Pablo, New York University executive vice president for Global Science and Technology and executive dean of the Tandon School of Engineering. “By directly visualizing how particles interact and reorganize as these materials yield, we can now connect nanoscale dynamics to large-scale mechanical behavior. This gives us a framework to design and tune the flow properties of soft materials with unprecedented precision.”
The results of this research were published in the Proceedings of the National Academy of Sciences.
Other contributors to this work include Miaoqi Chu, Zhang Jiang and Suresh Narayanan from Argonne, and Matthew Tirrell from Argonne and the University of Chicago.
This study was funded by DOE Office of Science, Basic Energy Sciences.
—This article was originally posted on the Argonne National Laboratory website