At its most important, science often challenges logic. And the new discovery may be the most absurd of all: water breaks.
A recent paper in Physical Review Letters, “Unexpected Fracture of Solids in Soft Liquids,” reports exactly that—when liquids are stretched with enough force, a supposedly liquid material breaks like a solid. Research suggests that viscosity, or a liquid’s resistance to flow, may play a bigger role in the mechanical properties of fluids than we previously believed. Moreover, this behavior applies to common fluids, such as water or oil, which raises new concepts for managing water in different applications.
“What we saw was so unexpected that we needed to repeat the experiment several times to make sure it was real,” Nicolas Alvarez, co-author and an engineer at Drexel University, said in a statement. “This really changes our understanding of fluid dynamics.”
It’s not about the equipment
A fancy way of explaining fractures is to talk about the balance between surface tension and the release of elastic energy that creates a new shape. More simply, fracture occurs when the energy released by the material exceeds the energy required to form a new surface. This is a defining feature that “describes solids” but not droplets, which “generate and flow rather than deform under stress,” according to the paper.
Researchers initially studied this process of yielding and flowing in a hydrocarbon-like mixture. What they expected to see was something like “the misbehavior common to anyone who puts honey in a cup of tea,” according to the statement. Then they heard a noise.
“The fracture made a very loud noise that startled me,” recalled Thamires Lima, the study’s lead author and a Drexel engineer. “At first I thought the machine was broken but soon I realized the noise was coming from the stretching fluid.”
Cracking cracks
According to Lima, that noise led the team to develop a “completely different scientific endeavor.” Once they confirmed that the noise was not caused by equipment failure, the researchers conducted experiments to examine the same behavior in liquids with the same viscosity as hydrocarbons. A high-speed camera recorded each session, giving the team a detailed view of how things unfolded.
Interestingly, the water showed a steady state of stretching until it reached “critical stress”, where it only broke in half. This level could be up to two megapascals, equivalent to “the pressure you would experience in an uncomfortable way if you were to push a 10-brick laundry bag up a hill and its string grazed your fingernail,” they explained.
This trend continued even when changes in temperature changed the viscosity, the team reported. The fluid fracture remained constant at 2 megapascals until the viscosity of each sample dropped so low that the tool, which has limited stretching capacity, could not stretch it further.
Liquid at the point of explosion
The findings challenge the consensus that fracturing is a property of elasticity, or a material’s ability to sustain stress. Until now, scientists believed that elasticity only applied to solids or to liquids that were cooled enough to become solid. But the new study shows that soft drinks with sufficient viscosity “are sufficient to promote solid behavior such as breakage,” Lima said.
Next, the researchers plan to identify the exact mechanisms of the body after their unexpected discovery. Another theory is cavitation, which refers to the rapid formation and collapse of vapor bubble shockwaves in liquids.
More importantly, the team believes the machine is good enough to work on other soft drinks. If true, that could provide new insights for engineers who use fluids “in everything from hydraulics to 3D printers to blood vessels,” the team said.
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