Oobleck Impacts Meet and Defy Expectations
A raindrop hitting a windshield spreads and splashes like an ordinary liquid. By contrast, a pool of cornstarch suspension, often referred to as Oobleck, can feel almost solid when stomped on. But what happens when these two responses to impact—spreading and so-called shear thickening—meet? The question is challenging because the ultrafast dynamics of shear thickening is hard to observe in conventional experiments. Now Anahita Mobaseri at the University of Minnesota and her colleagues have found answers by using high-speed imaging and force measurements to capture how drops of Oobleck act when they strike a surface [1]. The findings could help engineers better control complex fluids in 3D printing, industrial coating, and other practical applications.
One might expect two possible behaviors for an Oobleck drop impacting a surface. If the drop is dilute, it should flow like a liquid because its cornstarch particles can easily move past each other. And if the drop is concentrated, it should promptly stiffen and behave as a solid pellet as its particles jam together through a process known as dilatancy.
In addition to these expected behaviors, Mobaseri and her colleagues identified a surprising third one associated with dense drops possessing a high characteristic shear rate—that is, the ratio of the drop’s impact velocity to its initial diameter. Before stiffening into a solid, these drops briefly spread like an ordinary liquid. The researchers found that this behavior arose from the interplay of dilatancy and fluid inertia, the tendency for fluids to keep moving.
–Ryan Wilkinson
Ryan Wilkinson is a Corresponding Editor for Physics Magazine based in Durham, UK.
References
- A. Mobaseri et al., “Inertia-dilatancy interplay governs shear thickening drop impact,” Phys. Rev. Lett. 136, 148201 (2026).