Drexel Researchers Discover Liquids Have a Breaking Point

While studying a tar-like simple liquid, Drexel researchers discovered that stretching it with enough force caused it to break apart like a solid object. Their findings suggest this phenomenon could occur in all simple liquids.

In a development that could shift our basic understanding of fluid mechanics, researchers from Drexel University have reported that, given the right circumstances, it is possible to induce a simple liquid to fracture like a solid object. Recently published in the journal Physical Review Letters, the research shows how viscous liquids can suddenly break if stretched with enough force.

The fracturing behavior suggests that viscosity — a liquid’s resistance to flowing — may play a more prominent role in its mechanical properties than previously understood. It also raises new possibilities for how liquids might be manipulated in everything from hydraulics to 3D printers to blood vessels.

“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 facture like a solid. And this is likely true for all simple liquids, including common examples, such as water and oil,” said Thamires Lima, PhD, an assistant research professor in Drexel’s College of Engineering, who helped to lead the research. “This fundamentally changes our understanding of fluid dynamics.”

The unexpected discovery happened while Lima and her collaborators were measuring the properties of two simple liquids as part of research with ExxonMobil Technology & Engineering Company. In the process of performing an extensional rheology test — a measure of how much force it takes to make a liquid flow — the tar-like liquids surprisingly separated with a sudden snap, rather than the drawn-out thinning behavior familiar to anyone who’s dolloped a glob of honey into a cup of tea.

“What we observed was so unexpected that we needed to repeat the experiments a few more times to make sure it was real,” said Nicolas Alvarez, PhD, 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.”

Recording the test with a high-speed camera enabled the team to observe a behavior that typically happens when a solid material, like a piece of metal, is put under tension. At a certain point, it begins to stretch, until reaching a point of critical stress where it suddenly breaks in half. This is called brittle fracture and according to the researchers, it had never before been observed in a simple liquid.

“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.”

The first liquids they observed fracturing were tar-like hydrocarbon blends, which fractured under a critical stress of 2 megaPascals — roughly the force of tension you’d unpleasantly experience if you pushed a laundry bag containing 10 bricks off a ledge and its drawstring snagged on your fingernail.

To understand this phenomenon, the group repeated the test with a different simple liquid, styrene oligomer, with the same viscosity as the hydrocarbon blends. Surprisingly, it fractured under the same stretching rate, suggesting that viscosity plays a key role in the liquids’ solid-like fracturing behavior and that all simple liquids may have the same breaking point.

This was further confirmed when the team repeated the tests at a series of different temperatures, to change the viscosity of the liquids. At each viscosity, there was a unique stretching rate that induced fracturing — always proportional to the 2 megaPascal “critical stress” point. Ultimately, each sample reached a low enough viscosity where the test equipment, which was limited in its stretching rate, could not break it.

The finding is significant because, until now, scientists have viewed fracturing as a property of elasticity — a material’s ability to hold stress. In liquid form, simple fluids do not have a dominant elastic mechanism to store stress, so when they’re pushed or pulled, liquids flow, rather than bend, or — until now — break.

In simple liquids, the concept of elasticity is largely irrelevant until the liquid is cooled below its “glass transition,” the temperature at which it starts becoming solid-like. Thus, fracturing a simple liquid — while still in its liquid state — clearly means that the fracture phenomenon is not restricted to elastic materials, according to the researchers.

“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. “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.”

The team also took the step of comparing a simple liquid — oligomer styrene liquid — to its polymer liquid counterpart. Their testing revealed that both liquids break at the same critical stress point, which suggests that elasticity actually does not play a role in the phenomenon of fracture in a simple liquid.

“This suggests that many other elastic liquids might also break at a relatively similar critical stress point,” Lima said. “This points to a phenomenon that is relatively chemistry independent and possibly generalizable to a wide range of liquids.”

The team plans to continue exploring the liquid fracture phenomenon in order to understand the physical mechanisms that produce it. Early clues suggested to the researchers that it could be related to cavitation — a stress reaction involving the formation and rapid collapse of vapor bubbles sending shockwaves through a liquid.

“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.”

In addition to Lima and Alvarez, Stuart E. Smith, Kazem V. Edmond, Manesh Gopinadhan, and Emmanuel Ulysse, from ExxonMobile Technology & Engineering Company, contributed to this research.

Read the full paper here: https://journals.aps.org/prl/accepted/10.1103/t2vy-32wr