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We Know Simple Fluids Can Flow. Turns Out, Some Can Fracture
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A team of researchers at Drexel University has discovered that simple, nonelastic fluids can fracture under stress, challenging long-held assumptions in fluid dynamics. The finding, led by chemical engineering professor Nicolas J. Alvarez and research professor Thamires Lima, emerged during an extensional rheology experiment with a hydrocarbon blend similar to crude oil. When stretched between metal plates, the viscous liquid emitted a sharp cracking sound instead of flowing smoothly, indicating a brittle fracture typically seen in solids like glass.
Conventional physics holds that simple fluids lack the elasticity necessary for fracture. Unlike complex viscoelastic fluidsāsuch as polymer melts with entangled molecular chainsāsimple fluids are expected to deform and flow under stress. Lima's observation initially seemed implausible. 'I thought it was the machine,' she recalled, but repeated tests confirmed the phenomenon, with each stretch producing a loud pop and camera footage revealing a classic brittle crack.
The researchers traced the cracking mechanism to nanoscale defects within the fluid. In brittle solids, imperfections at the tens-of-nanometers scale grow catastrophically under critical stress. The hydrocarbon fluid, despite minimal elasticity, exhibited identical behavior. 'Nobody expected that this would be possible in this kind of simple fluid,' said Arnold Mathijssen, a fluid physicist at the University of Pennsylvania, noting that viscosity usually allows molecular rearrangement rather than crack formation.
This discovery overturns the prevailing theory linking liquid fracture to elasticity. In 2016, Alvarez and colleagues showed that complex fluids like molten polystyrene could fracture, but they attributed that to elastic properties. The new experiment, however, eliminates elasticity as a prerequisite. 'If there is no elasticity in a problem, then how can you think about initiation or growth of a crack?' asked Brato Chakrabarti, a physicist at the International Center for Theoretical Sciences in Bengaluru, India.
The implications extend beyond fundamental science. The hydrocarbon blend studied is similar to industrial fluids used in oil extraction and polymer processing. Understanding fracture in such liquids could lead to safer handling, improved pipeline design, and enhanced manufacturing techniques. 'Every time that she measured it, the material would break,' Alvarez said, emphasizing the reliability and significance of the finding.
Moving forward, the team plans to investigate the molecular conditions that trigger fracture in simple fluids. They aim to develop a theoretical framework to predict crack initiation and growth, potentially revising textbooks on fluid mechanics. 'We thought elasticity was something that was a prerequisite for such solid-type breaking,' Alvarez reflected. 'But this shows we need to rethink that.'
