Simple Fluids Can Fracture: A New Physics for Engineers

A team at Drexel University has shown that simple, non-elastic fluids can undergo brittle fracture—a phenomenon previously thought exclusive to solids or complex viscoelastic liquids. The discovery, published in collaboration with Exxon Mobil, upends decades of fluid dynamics theory and has direct implications for industrial processes like inkjet printing, fiber spinning, and even brain injury protection.

The Discovery: A Crack in the Liquid

Thamires Lima, a research professor at Drexel, was using extensional rheology to test a hydrocarbon blend for Exxon Mobil when she heard a sharp crack. The fluid—a simple, non-elastic mixture—had snapped apart under tension. “I thought it was the machine,” Lima said. But repeated experiments confirmed the fluid was fracturing, producing a loud pop like a snapped rubber band.

High-speed cameras revealed the break was a classic brittle fracture, identical to what happens when glass or porcelain shatters. The crack nucleated at a microscopic defect and propagated catastrophically. The critical stress at fracture was consistently 2 megapascals across both simple and complex fluids tested.

Why It Matters: Speed and Mechanism

In prior work (Alvarez et al., 2016, Physical Review Letters), complex fluids like melted polystyrene fractured at crack speeds of about 0.07 m/s. In the simple hydrocarbon blend, cracks tore through at 500 to 1500 m/s—thousands of times faster. The reason: simple fluids lack the long molecular chains that absorb energy in complex fluids. “There’s really nothing to slow that crack down,” said Nicolas J. Alvarez, the lab lead.

The fracture mechanism appears to involve cavitation—the formation of tiny bubbles under stress. In simple fluids, these bubbles can coalesce into a crack if formed rapidly enough. This aligns with predictions from Daniel D. Joseph’s 1995 and 1998 papers, which argued that any liquid could fracture under sufficient tearing stress.

The Parameter: Critical Stress and Viscosity

The team varied the temperature of the hydrocarbon blend to change its viscosity. Only the least viscous sample failed to fracture—likely because the machine’s maximum pull speed (500 mm/s) wasn’t fast enough to trigger cavitation. Lima suspects that with a faster apparatus, even honey or water could be fractured.

Crucially, the critical stress of 2 MPa was proportional to the product of viscosity and strain rate. This suggests a universal criterion for liquid fracture, independent of elasticity.

Implications for Engineers

For developers working on computational fluid dynamics (CFD) or material processing, this discovery means existing models for liquid behavior under high strain may need revision. For example:

  • Inkjet printing: Droplet formation involves rapid extension. If the fluid fractures, print quality degrades.
  • Fiber spinning: Melt spinning of polymers could experience unexpected breakage if the fluid is too simple.
  • Soft robotics and brain injury: Understanding fracture thresholds could improve protective gear design.

What’s Next

The Drexel team plans to use transparent fluids to visualize crack formation in real time, and to freeze fractured surfaces for nanoscale microscopy. Alvarez is also exploring how this phenomenon affects fiber spinning processes. For the broader physics community, the finding challenges the assumption that elasticity is a prerequisite for fracture in liquids.

Key takeaway: If you’re working with high-speed fluid processes, don’t assume your simple fluid will just flow—it might snap.