GoKawiilThamires Lima, a research professor in chemical engineering at Drexel University, studies the properties of thick, viscous liquids — think honey or molasses, though in a lab you’re more likely to find polypropylene or crude oil. Using a method called extensional rheology, Lima stretches liquids between metal plates to find the force that makes them flow.
A few years ago, she was conducting a test as part of a project in collaboration with the oil and gas company Exxon Mobil when she heard a short, sharp crack. “I thought it was the machine,” Lima said. But the crack came from the fluid that the machine was pulling: a gooey, black blend of hydrogen and carbon. Instead of stretching, the fluid had fractured.
Fractures are known to occur in certain elastic complex fluids, which can act like solids under certain conditions. But Lima was working with a nonelastic simple fluid. Even with almost no elasticity, it snapped apart under stress.
“Nobody expected that this would be possible in this kind of simple fluid because viscosity usually just rearranges the molecules,” said Arnold Mathijssen, a fluid physicist at the University of Pennsylvania. “You don’t expect it to crack. But it does, so I think that’s what’s really surprising.”
A Brittle Break
Lima stretched the liquid again and again to prove that the unexpected crack wasn’t a one-off. “Every time that she measured it, the material would break,” said Nicolas J. Alvarez, the professor of chemical engineering at Drexel University whose lab led the research. “It makes a loud pop. I mean, like you just took a rubber band and pulled it and stretched it and it snapped.”
Thamires Lima , a research professor in chemical engineering at Drexel University, was stretching a liquid in an extensional rheometer when she heard a short, sharp crack. Courtesy of Thamires Lima
Convinced the snap wasn’t a fluke, Lima and Alvarez used high-speed cameras to look at the phenomenon more closely. They realized that the break was essentially a “brittle fracture,” the kind you might see when you drop a dish made of glass or porcelain.
Brittle fractures happen to brittle solids, which have elasticity. Apply some stress to glass or porcelain and it deforms a very tiny bit, and then — if you don’t push it past its breaking point — it springs back to normal once the stress is removed. However, solids are never perfect. In most cases, a brittle solid will have a teeny, tiny defect — a crack at the scale of tens of nanometers. Once the solid is stressed past a critical point, it becomes energetically more favorable for the solid to grow the crack than to elastically store the stress. At that point, the crack grows catastrophically, rapidly breaking the solid apart.
Some complex fluids, called viscoelastic liquids, also have elasticity. For example, polymer melts — melted versions of the polymers in plastics — are made up of long chains of molecules, which become entangled with one another and increase the material’s elastic component.
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