In a surprising study that could revolutionize fluid mechanics, researchers at Drexel University have shown that under certain conditions, a simple liquid can behave like a solid and fracture. Study, published in Physical Examination Lettersshows that viscous liquids can burst suddenly when stretched with sufficient force.
This finding suggests that viscosity, or a liquid’s resistance to flow, plays a larger role in its mechanical performance than scientists previously believed. It also opens the door to new ways to control fluids in applications from hydraulics and 3D printing to blood flow in the body.
“Our research shows that if they are pulled with sufficient force in each area, a simple fluid – a fluid that flows – will reach what we call ‘critical stress,’ when it will actually break like a solid. And this may be true for all simple fluids, including common examples, such as water and oil,” said Thamires Lima, PhD, assistant research professor at the College of Drexel, who was an assistant research professor at Drexel. “This dramatically changes our understanding of fluid dynamics.”
Amazing snap during testing
The discovery came unexpectedly when Lima and his team were studying two soft drinks in collaboration with ExxonMobil Technology & Engineering Company. During the analysis of extended rheology – which measures how much force is needed to make water flow – the researchers noticed something unusual. Instead of stretching and thinning like honey, the tar-like liquids suddenly erupted.
“What we saw was so unexpected that we needed to repeat the experiment several times to make sure it was true,” said Nicolas Alvarez, PhD, a professor in the College of Engineering whose lab led the research. “Once we confirmed this phenomenon, the investigation became a completely different scientific endeavor.”
Using a high-speed camera, the team captured behavior typically seen in solid materials. When stretched, solids expand until they reach a critical stress level, and then break suddenly in a process known as brittle fracture. According to the researchers, this type of fracture had never been seen in a simple liquid.
“This was really amazing to see,” Lima said. “The break made a very loud noise that scared me. I thought at first that the machine was broken, but soon I realized that the noise was coming from the stretching fluid.”
Viscosity and Dissolving Point
The first liquids to exhibit this behavior were hydrocarbon compounds such as tar, which broke down at a stress of 2 megaPascals – about the force you would experience when a laundry bag filled with 10 bricks was stuck to your finger when you fell.
To test further, the researchers tested another simple liquid, a styrene oligomer, with a similar viscosity. It broke under the same stretching conditions, indicating that viscosity is an important factor in this solid-like breaking behavior and suggesting that many soft drinks may share the same breaking point.
The team then changes the temperature to change the viscosity. In each case, they found a certain rate of stretching that caused the fracture, always tied to the same pressure of 2 megaPascal. At low viscosities, water would not break because the test equipment could not stretch it fast enough.
Amazing Long Term Ideas
Until now, fracture has been considered as a property of elasticity, which is the ability of the material to store and withstand stress. However, soft drinks do not maintain stress in this way. Instead, they flow when force is applied, rather than bending or breaking.
In most cases, elasticity becomes important only when the liquid is cooled below the “glass transition,” where it begins to behave like a solid. Seeing the fracture of wet materials that are still completely fluid shows that fracture is not limited to elastic materials.
“Although alcoholic beverages and polymers – things like Oobleck or homemade slime – have shown solid behavior such as breaking, soft water has always been thought to show a change in temperature above its glass transition and therefore will not break,” Lima said. “Showing that viscous effects are sufficient to promote solid fracture-like behavior opens up a world of new questions to be explored in this area of scientific research.”
A Wide and Unexpected Event
The researchers also compared a simple liquid, oligomer styrene, and a polymer-based liquid. Both broke at the same stress point, suggesting that elasticity is not responsible for the fracture behavior of soft water.
“This suggests that many other liquids can also break down at a similar time to stress,” Lima said. “This indicates a relatively independent chemical phenomenon and may be general to many types of alcoholic beverages.”
What Causes Liquids to Disintegrate
The team plans to further investigate why this is happening and how widespread the effect may be. Preliminary evidence points to cavitation — a process where tiny vapor bubbles form and collapse rapidly, creating waves in a liquid — as a possible explanation.
“Now that we have documented this unexpected behavior, the work to fully understand why it occurs and how the behavior manifests itself in other beverages is an important next step,” Lima said. “It will also be interesting to see how this can be used to aid fiber spinning and other applications that use viscous liquids.”
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