Oobleck is a non-Newtonian fluid. Oobleck is a suspension or a substance that can mimic the qualities of a solid or a liquid.
It flows like milk when gently stirred but turns rock-solid when impacted at high speed. This fascinating phenomenon, known as shear-thickening, results in spectacular demonstrations like running on a pool of Oobleck without submerging into it, as long as the runner doesn’t stop.
Scientists from Aix-Marseille University in France have now studied the regular and prominent surface waves that form when an Oobleck flows down an inclined slope. Similar waves can be observed on gutters and windows on rainy days. However, the scientists noted qualitative differences with water waves; waves in Oobleck grow and saturate much faster. To unveil Oobleck waves’ origin, they conducted careful experiments with a mixture of cornstarch and water down an inclined plane.
Using controlled perturbation of the flow, scientists measured the onset of wave appearance and their speed. They used the laser to estimate the fluid film thickness.
These experiments revealed that for concentrated Oobleck, the onset of destabilization is different for destabilization in a Newtonian fluid such as water.
This surprising observation led the team to look for a scenario to explain their formation.
Under effect, as appeared by ongoing examinations, Oobleck suddenly changes from liquid to solid due to the initiation of frictional contacts between the starch particles. When streaming down a slope, this proliferation of frictional contacts prompts inquisitive conduct: The suspension’s flow velocity decreased when the imposed stress increased. Specialists have demonstrated that this effect couples to the flow free surface and can immediately produce a regular wave pattern.
The proposed mechanism is generic. These findings could thus provide new grounds to understand other flow instabilities observed in various configurations, particularly in industrial processes facing problematic flow instabilities when conveying Oobleck-like materials such as concrete, chocolate, or vinyl materials.
- Baptiste Darbois Texier et al. Surface-wave instability without inertia in shear-thickening suspensions, Communications Physics (2020). DOI: 10.1038/s42005-020-00500-4