Researchers from Drexel University have discovered that liquids can, it appears, fracture like solids under specific conditions. In a discovery that could shift our understanding of fluid mechanics, it now appears that viscous fluids can suddenly break when stretched with enough force.
Unlike solids, which will stretch and eventually snap, liquids have never been believed to have a breaking point. This new research, however, appears to challenge this belief.
“Our findings show that if pulled apart with enough force per area, a simple liquid — a liquid that flows — will reach what we call a point of ‘critical stress,’ when it will actually fracture like a solid,” said Thamires Lima, Ph.D., an assistant research professor in Drexel’s College of Engineering, who helped to lead the research.
Interestingly, the team also found that the liquid “snapping” produces a sudden noise, like that of breaking solids. Needless to say, this was also not expected.
“What we observed was so unexpected that we needed to repeat the experiments a few more times to make sure it was real,” explained Nicolas Alvarez, Ph.D., a professor in the College of Engineering whose lab led the research.
“Once we confirmed the phenomenon, the research became an entirely different scientific endeavor,” he added.
Liquids can break after all
“This was an incredibly surprising thing to behold,” Lima said. “The fracture caused a very loud snapping noise that actually startled me. I thought at first the machine had broken, but soon realized that the noise came from the stretching fluid,” she said.
The critical element to the research is that liquids will only break like this when pulled apart at speeds that prevent them from “flowing away” from stress. Like in solids, this provides enough time for stress to build up enough for a sudden fracture to form.
From experimentation, the team found that liquids tend to break at around 2 megapascals of tension. The exact figure, of course, depends on the liquid’s viscosity (thickness).
The higher the viscosity, the easier it appears it can snap. “This is likely true for all simple liquids, including common examples, such as water and oil… This fundamentally changes our understanding of fluid dynamics,” she added.
“Although viscoelastic and polymer liquids — things like Oobleck or homemade slime — have demonstrated solid-like fracture behavior, simple liquids have always been thought to exhibit continuous deformation at temperatures above their glass transition and therefore would not fracture,” Lima said.
As for what this means for the real-world outside of a lab, it could open some interesting lines of research in various industries. “Showing that viscous effects are enough to promote solid-like fracture behavior opens a world of new questions to explore in this area of scientific inquiry,” she said.
Interesting future study possibilities
For example, 3D printing, which uses liquid polymers, could benefit from a better understanding of liquid fracture limits. Fiber manufacturing, which tends to stretch liquids into threads, would clearly also benefit from such insights.
In medical sciences, things like blood (which are liquids) could be investigated to see if it fractures under certain conditions and find ways to avoid it. Hydraulics could also be another interesting area of study.
“This suggests that many other elastic liquids might also break at a relatively similar critical stress point,” Lima added. “This points to a phenomenon that is relatively chemistry independent and possibly generalizable to a wide range of liquids,” she explains.
Another interesting area of study could be cavitation from boat or submarine propellers, pumps, and some sonar systems. Here, tiny bubbles form and then collapse violently, which can damage metals, create shockwaves, and make noise.
With a better understanding of the physics going on, it might be possible to limit or even eliminate cavitation issues in the future.
“Now that we have reported this unanticipated behavior, the work of fully understanding why it happens and how the behavior manifests in other liquids is an important next step,” Lima said. “It will also be interesting to see how this finding may be applied to assist fiber spinning and other applications that use viscous liquids,” she added.
You can view the study for yourself in the journal Physical Review Letters.