The Discovery That Challenges Fluid Dynamics Fundamentals

Researchers at Drexel University have uncovered a phenomenon that upends centuries-old assumptions about how liquids behave. In a study published in Physical Review Letters, the team demonstrated that simple liquids—those that flow freely to fill their containers—can fracture like brittle solids when subjected to sufficient tensile stress. This finding, achieved through precise extensional rheology experiments, reveals that viscosity alone can drive solid-like breaking behavior, independent of elasticity.

Traditionally, fracture has been the domain of solids, where atomic bonds snap under stress. Liquids, by contrast, were thought to deform continuously without breaking, as long as they remain above their glass transition temperature. Drexel's work shows this isn't always true. By stretching a tar-like hydrocarbon liquid at high strain rates, the material suddenly snapped with a loud crack, captured on high-speed video. The critical stress threshold hovered around 2 megapascals (MPa)—roughly the force of suspending a laundry bag with 10 bricks on a snagged drawstring.

This breakthrough emerged from routine tests in Professor Nicolas Alvarez's lab in Drexel's College of Engineering. Assistant Research Professor Thamires Lima recalls the shock: "The fracture caused a very loud snapping noise that actually startled me. I thought at first the machine had broken." Repeated trials confirmed the result, shifting the project from verification to exploration.

Background: Redefining Liquid vs. Solid Mechanics

Fluid mechanics has long distinguished simple liquids (e.g., water, oil) from viscoelastic materials like slime or Oobleck, which can temporarily stiffen under stress. Viscoelastic fluids fracture when deformed rapidly enough for their storage modulus (G') to match the loss modulus (G"), mimicking solids. But simple liquids lack the polymeric chains responsible for elasticity, so textbooks predicted endless flow, not fracture.

Drexel's experiment used squalane-like hydrocarbons, chosen for their Newtonian flow—constant viscosity regardless of shear rate. By applying uniaxial extension via capillary breakup extensional rheometry (CaBER), the team ramped up strain rates. At viscosities around 10,000 to 100,000 Pa·s, the liquid neck thinned until it catastrophically fractured, forming sharp cracks rather than smooth necking.

Step-by-step, the process unfolds:

• Initial stretching: Liquid filament forms between plates, thins uniformly.
• Critical strain rate: Proportional to viscosity, reaches ~2 MPa tension.
• Fracture initiation: Cavitation bubbles nucleate, grow, and collapse, propagating cracks.
• Brittle snap: Clean break with minimal plastic deformation, audible pop.

Tests across temperatures (altering viscosity) showed consistent fracturing until equipment limits prevented high enough rates at low viscosity.

Drexel's Experimental Setup and Methodology

The core apparatus was a custom CaBER device, pulling liquid bridges between rotating cylinders or plates. High-speed cameras (up to 10,000 fps) recorded the necking dynamics. Collaboration with ExxonMobil provided precise viscosity measurements and additional liquids like styrene oligomer—a monomer matching the hydrocarbon's viscosity but with a polymeric counterpart for elasticity comparison.

Key parameters:

Both simple and polymer versions fractured identically, isolating viscosity as the driver. Lima notes: "Viscous effects are enough to promote solid-like fracture behavior."

Potential mechanism: Cavitation, where tension creates vapor voids that implode, generating shockwaves akin to solid fracture nucleation.

Key Findings: Viscosity-Driven Brittle Fracture

The hallmark result: A universal critical stress of ~2 MPa triggers fracture across tested liquids, regardless of chemistry. Strain rate scales with viscosity η via Hencky strain rate ε̇ ∝ 1/η, ensuring consistent stress σ = 3ηε̇.

Unlike viscoelastic fracture (tied to Deborah number De = λε̇ >1, where λ is relaxation time), this occurs in Newtonian fluids (De≈0). Implications challenge linear viscous flow models, suggesting nonlinear instabilities at high extensions.

Alvarez emphasizes: "This fundamentally changes our understanding of fluid dynamics."

Researchers Behind the Drexel Breakthrough

Led by Nicolas J. Alvarez (Professor, Chemical and Biological Engineering) and Thamires Lima (Assistant Research Professor), the team included ExxonMobil's Stuart E. Smith, Kazem V. Edmond, Manesh Gopinadhan, and Emmanuel Ulysse. Alvarez's lab focuses on rheology for industrial applications; Lima bridges engineering and materials science.

This interdisciplinary effort highlights Drexel's strength in collaborative research, blending academia with industry like ExxonMobil for real-world relevance.

Implications for Materials Science and Engineering

This discovery reframes liquids' mechanical limits. In fiber spinning (e.g., textiles, optics), unexpected fractures could optimize processes. Hydraulic fracturing in oil/gas might benefit from controlled cavitation. 3D printing viscous inks gains precision by predicting break points.

Biologically, blood or synovial fluid under extreme tension (e.g., cavitation in joints) could explain pathologies. Geological contexts like magma or groundwater fracturing gain new models.Read the full paper here.

Broader Applications Across Industries

• Manufacturing: Predict die swell or draw resonance in polymer extrusion analogs.
• Energy: Enhance fracking efficiency by harnessing viscous fracture.
• Biomedicine: Model embolisms or drug delivery in viscous media.
• Geophysics: Simulate fluid-driven earthquakes or volcanic eruptions.

Lima envisions: "It will also be interesting to see how this finding may be applied to assist fiber spinning."

Challenges and Future Research Directions

Open questions: Exact cavitation role? Universality to water/oil? Molecular-scale dynamics? Drexel plans atomic simulations and broader liquid tests. Alvarez: "The work of fully understanding why it happens... is an important next step."

This positions Drexel at forefront of soft matter physics, attracting funding and talent.

Drexel's Role in Advancing Higher Education Research

Drexel's interdisciplinary ethos—spanning Arts & Sciences, Engineering—fosters such surprises. As a US leader in materials research, it exemplifies how universities drive innovation amid funding pressures. Explore Drexel faculty openings via higher ed faculty jobs.

Stakeholder Perspectives and Global Reactions

Industry (ExxonMobil) praises viscosity insights for processes. Peers call it "shocking," per social buzz. No controversy; consensus on paradigm shift.

Future Outlook: Reshaping Liquid Mechanics

This Drexel breakthrough promises refined models, safer designs, novel tech. As research expands, expect textbooks rewritten, processes revolutionized—cementing Drexel's legacy in US higher ed innovation.

Photo by Amin Zabardast on Unsplash