In a surprising discovery that could reshape fluid mechanics, Drexel University researchers have shown that under certain conditions, simple liquids can behave like solids and fracture. This research physical review letterindicating that a viscous liquid can suddenly break if stretched with enough force.
The discovery suggests that viscosity, or resistance to fluid flow, plays a much larger role in mechanical behavior than scientists previously thought. It also opens the door to new ways to control fluids in a variety of applications, from hydraulics and 3D printing to blood flow in the body.
“Our results show that when pulled apart with enough force per area, simple liquids, flowing liquids, reach a point we call ‘critical stress’ and actually break down like solids. And this is probably true for all simple liquids, including common examples like water and oil,” said Tamirez Lima, Ph.D., assistant professor at Drexel Institute of Technology, who led the study. “This fundamentally changes our understanding of fluid mechanics.”
Amazing snaps during testing
The discovery came unexpectedly when Lima and her team were working with ExxonMobil Technology & Engineering Company to study two simple liquids. During an extensional rheology test, which measures how much force is required to create a flow of liquid, the researchers observed something unusual. The tarry liquid suddenly fell apart, instead of stretching and thinning like honey.
“What we observed was so unexpected that we needed to repeat the experiment several more times to confirm it was real,” said Dr. Nicolas Alvarez, a professor in the School of Engineering who led the study. “Once the phenomenon was confirmed, the study became a completely different scientific endeavor.”
The researchers used a high-speed camera to photograph behaviors commonly seen in solid materials. When a solid is stretched, it stretches until it reaches a critical stress point and then breaks suddenly in a process known as brittle fracture. According to the researchers, this type of destruction has never been observed before in simple liquids.
“This was incredibly surprising,” Lima said. “When the fracture occurred, there was a very loud crackling sound that actually startled me. At first I thought the machine had broken, but I soon realized that the sound was coming from the stretching fluid.”
Viscosity and critical breaking point
The first liquid to exhibit this behavior was a tar-like hydrocarbon mixture that fractured at a critical stress of 2 megapascals. This stress is about the same as the force a laundry bag filled with 10 bricks might feel if it catches your fingernail while falling.
To investigate further, the researchers tested another simple liquid with the same viscosity: styrene oligomers. It failed under the same stretching conditions, indicating that viscosity is a key factor in this solid-like failure behavior and suggesting that many simple liquids may share similar failure points.
The team then adjusted the temperature to change the viscosity. We found that at each level, the specific extension rate that causes failure is always associated with the same critical stress of 2 megapascals. If the viscosity was low, the test equipment could not stretch the liquid fast enough to break it up.
Challenging long-held assumptions
Traditionally, fracture has been thought of as an elastic property, the ability of a material to accumulate and withstand stress. However, simple liquids usually do not accumulate stress in this way. Instead, it flows when force is applied rather than bending or breaking.
In most cases, elasticity is only relevant when the liquid is cooled below the “glass transition point”, where it begins to behave more like a solid. Observing the fracture of a liquid that is still in its fully liquid state shows that fracture is not limited to elastic materials.
“While viscoelastic and polymeric liquids such as oobleck and homemade slime exhibit solid-like fracture behavior, it has always been thought that simple liquids do not fracture because they exhibit continuous deformation at temperatures above their glass transition temperature,” Lima said. “Showing that viscous effects are sufficient to promote solid-like fracture behavior opens up a world of new questions to explore in this area of scientific research.”
A wide range of unexpected phenomena
The researchers also compared a simple liquid, oligomeric styrene, to a related polymeric liquid. Both failed at the same critical stress point. This suggests that elasticity is not responsible for the fracture behavior in simple liquids.
“This suggests that many other elastic liquids may also fail at relatively similar critical stress points,” Lima said. “This demonstrates a phenomenon that is relatively chemistry-independent and probably generalizable to a wide range of liquids.”
What causes liquids to break down
The research team will continue to investigate why this happens and how widespread the effects are. Early evidence points to cavitation, a process in which small vapor bubbles form and rapidly collapse, creating shock waves within the liquid, as a possible explanation.
“Now that we have reported this unexpected behavior, an important next step is to fully understand why it occurs and how it manifests in other liquids,” Lima said. “It will also be interesting to see how this discovery is applied to support fiber spinning and other applications using viscous liquids.”
