Almost everyone has experienced the frustration of trying to get ketchup out of a bottle. At first, nothing happens.
The ketchup seems almost solid and refuses to move. Then, suddenly, it starts flowing, sometimes too quickly and all at once. Scientists call this surprising change in behavior “yielding.”
Yielding happens when a material switches from acting like a solid to behaving like a liquid. It is not unique to ketchup.
Many everyday materials, including toothpaste, paint, concrete, and even some materials used in advanced batteries and 3D printing, go through the same transition.
Despite how common this behavior is, scientists have struggled to understand exactly what causes it.
The difficulty comes from the fact that these materials are often dense and opaque, making it nearly impossible to observe what is happening inside them.
Researchers from Argonne National Laboratory and the University of Chicago have now uncovered important clues.
Using powerful X-ray beams and advanced computer simulations, they tracked the behavior of materials similar to ketchup as they changed from solid-like to liquid-like states.
The researchers created two nearly identical materials, each made of tiny particles suspended in liquid. In one material, the particles mostly pushed away from one another. In the other, the scientists added salt, causing the particles to become slightly attractive and stick together weakly.
At first glance, both materials looked almost exactly the same. The differences only became visible when force was applied.
When the particles repelled each other, the material changed shape smoothly and evenly. It flowed in a predictable way without developing weak spots.
The material with slightly attractive particles behaved very differently. The particles clumped together, creating dense regions separated by small empty spaces. When stress was applied, some areas started moving while neighboring areas remained stuck. The material split into sections that flowed at different speeds. Scientists call these regions “shear bands.”
This led to unusual behaviors. Sometimes the material resisted movement for a period of time and then suddenly started flowing.
This phenomenon is known as delayed yielding. In other cases, the material flowed for a while and then unexpectedly became solid again, even though the applied force had not changed. This process is called resolidification.
These behaviors are important because they influence how products perform. Materials that suddenly thicken or stop flowing can clog manufacturing equipment or produce uneven products.
To understand what was happening inside the materials, the team combined traditional measurements of flow with a specialized X-ray technique that could monitor the movement of groups of particles in real time.
They then used high-performance computer simulations to track the motion of individual particles, something experiments alone could not do.
The simulations revealed that weak connections between flowing and non-flowing regions play a crucial role. Under small stresses, these connections remain intact and the material moves very slowly.
As stress continues, some connections suddenly break, allowing particles to slip past one another and causing delayed yielding. Over time, new connections form and lock the structure again, producing resolidification.
The researchers say their findings provide a bridge between what happens at the microscopic level and how materials behave on a larger scale.
This new understanding could help scientists and engineers design soft materials with precisely controlled flow properties, leading to better products and more reliable manufacturing processes in the future.
