Unlocking the secrets and techniques of oobleck—unusual stuff that’s each liquid and stable

Oobleck has lengthy been my favorite example of a non-Newtonian fluid, and I’m not alone. It is a vastly widespread “kitchen science” experiment as a result of it is easy and simple to make. Combine one half water to 2 elements corn starch, add a splash of meals coloring for enjoyable, and you have oobleck, which behaves as both a liquid or a stable, relying on how a lot stress is utilized. Stir it slowly and steadily, and it is a liquid. Punch it laborious, and it turns extra stable beneath your fist. You possibly can even fill small swimming pools with the stuff and stroll throughout it because the oobleck will harden each time you step down—a showy physics demo that naturally reveals up lots on YouTube.

The underlying physics ideas of this straightforward substance are surprisingly nuanced and complicated, and thus fascinating to scientists. Molecular engineers on the College of Chicago have used dense suspensions of piezoelectric nanoparticles to measure what is occurring on the molecular degree when oobleck transitions from liquid to stable conduct, in accordance with a new paper printed within the Proceedings of the Nationwide Academy of Sciences.

Towards the tip of his life, Isaac Newton laid out the properties of an “ultimate liquid.” A type of properties is viscosity, loosely outlined as how a lot friction/resistance there’s to stream in a given substance. The friction arises as a result of a flowing liquid is basically a collection of layers sliding previous each other. The quicker one layer slides over one other, the extra resistance there’s; the slower one layer slides over one other, the much less resistance there’s. However the world isn’t a super place.

In Newton’s ideal fluid, the viscosity largely depends upon temperature and strain: water will proceed to stream no matter different forces appearing upon it, corresponding to being stirred or combined. In a non-Newtonian fluid, the viscosity adjustments in response to an utilized pressure or shearing power, thereby straddling the boundary between liquid and stable conduct. Stirring a cup of water produces a shearing power, and the water shears to maneuver out of the best way. The viscosity stays unchanged. However for non-Newtonian fluids like oobleck, the viscosity adjustments when a shearing power is utilized.

Ketchup, as an example, is a shear-thickening non-Newtonian fluid, which is one cause smacking the underside of the bottle does not make the ketchup come out any quicker; the applying of power will increase the viscosity. Yogurt, gravy, mud, pudding, and thickened pie fillings are different examples. And so is oobleck. (The identify derives from a 1949 Dr. Seuss youngsters’s ebook, Bartholomew and the Oobleck.) Against this, non-drip paint displays a “shear-thinning” impact, brushing on simply however changing into extra viscous as soon as it is on the wall.

In 2019, MIT researchers developed a useful mathematical mannequin to foretell how oobleck goes from liquid to stable and again once more beneath completely different situations. They tailored their working mannequin for moist sand, a granular materials. There are some similarities, however the corn starch particles in oobleck are one-hundredth the dimensions of grains of sand (between 1 to 10 microns). At these small dimension scales, the physics is markedly completely different. For example, temperature has extra of an impression on corn starch particles, as do electrical prices, which construct up between particles to trigger a repulsion impact. So, whereas moist sand has the identical viscosity at any given packing density whatever the stress utilized (e.g., stirring or punching), oobleck’s viscosity adjustments dramatically.

The MIT staff specifically added a “clumpiness variable” to their mannequin, describing the quantity of frictional contact between corn starch particles versus lubricated contact to foretell how that new variable would change in response to completely different stresses. Then, they ran pc simulations of prior lab experiments—which concerned squeezing and shearing oobleck between two plates and capturing a simulated small projectile right into a tank of oobleck—to check the mannequin’s predictions. These simulations matched the experimental outcomes from the sooner research.