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.

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.

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.

When the samples were not under stress, they looked almost identical. 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 the University of Chicago. “That leads to more complex behavior, such as delayed yielding and resolidification, which you do not see in simple fluids.”

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.

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