Through thick and thin: Researchers solve decades-old fluid question

What do paint, dishwasher detergent, ketchup and blood have in common? All are composed of particles suspended in a carrier liquid and flow when stirred or forced, but remain thick or even gel-like at rest.

That very useful behavior in complex fluids is called shear thinning: their viscosity decreases during mixing and increases at rest. But when the mixing speed increases—as required in many large-scale industrial processes—certain fluids can pass through the region of shear thinning and move into a region where viscosity increases dramatically, making them difficult or impossible to stir. This effect, known as shear thickening, has been under investigation for several decades as engineers sought to solve complex production problems caused by the phenomenon.

Now, a team of nanoscientists and physicists from Argonne National Laboratory has unraveled this 30-year mystery by studying a shear-thickening fluid with X-rays. The study could lead to applications in 3-D printing, the chemical industry and the biomedical field.

In the late 1980s, scientist Richard L. Hoffman proposed a simple model: When fluids are mixed at low speeds, the suspended particles form ordered layers that can slide easily across each other, facilitating flow. But when exposed to high speeds, the layers become disordered and stumble over one another, hindering flow; this change in the type of flow is called “order-to-disorder transition.” It’s a bit like a disorderly crowd, pushing and shuffling its way through a congested exit.

Other researchers were able to observe this behavior in many fluids, but not in every shear-thickening fluid. So scientists proposed several other models to explain the shear-thickening phenomenon, but none of them address Hoffman’s model.

“So the puzzle remains: how is order-to-disorder of particles related to shear-thickening behavior? Why does it happen only in certain complex fluids?” said Xiao-Min Lin, a nanoscientist with a joint appointment at the University of Chicago James Franck Institute and the Center for Nanoscale Materials at Argonne.