What is it about?

The surprising behavior of shear-thickening suspensions, such as the familiar cornstarch–water mixture colloquially known as Oobleck, is the stuff of exciting public science demonstrations and popular science fair projects, but also has important practical implications for a wide range of industrial and geophysical flows. It has long been known that mixtures of cornstarch and water, like many other mixtures of fluids with solid particles, can transition rapidly from liquid like to solid like when pressure is applied to make part of the suspension flow (shear) rapidly. Here we use high speed imaging techniques to separate the movement of the cornstarch particles from the suspending fluid. We find that, most of the time, the two move together, but as the oobleck transitions to the solid phase, these periods of harmony are interrupted by brief bursts of dramatic differences in flow, with the fluid sometimes going in the complete opposite directions from the suspension as a whole.

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Why is it important?

Shear thickening has been the subject of extensive study for several decades, but much about the transition remains mysterious and we still don’t have a model good enough to predict where the transition will occur, or design materials that will produce desired thickening properties. The results presented here illustrate the complex structures associated with shear thickening and show how fluctuations combine to produce the large forces that result in dramatic increases in viscosity, and will help point the way towards a complete theory of shear thickening in suspensions like Oobleck.

Perspectives

It has been fascinating to visualize and precisely measure the complex structures associated with shear thickening and be able to show how the fluctuations combine to produce the large forces that result in dramatic increases in viscosity. It’s all the more exciting to be able to do that in a familiar and accessible material like Oobleck.

Jeffrey Urbach
Georgetown University

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This page is a summary of: Structure of propagating high-stress fronts in a shear-thickening suspension, Proceedings of the National Academy of Sciences, August 2022, Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.2203795119.
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