Q: How can oobleck act as both a solid and a liquid?
Though I know this demo by the name “cornstarch goop,” a lot of people call it “oobleck.” Oobleck apparently gets its name from the Dr. Seuss book Bartholomew and the Oobleck. In the story, oobleck was a sticky green substance that fell from the sky and causes chaos in a kingdom.
Real-world ooblek is a non-Newtonian fluid. “Normal” fluids and liquids are Newtonian, meaning their viscosity or flow only changes with temperature or pressure. Viscosity is a measure of its resistance to deformation at some rate. It’s basically a fluid’s “thickness.” For example, water is pretty “thin” and flows easily. Honey tends to flow more slowly because it’s “thicker.” The viscosity only changes with changes to temperature or pressure. For example, warm syrup will flow more easily than cold syrup.
The viscosity of non-Newtonian fluids, on the other hand, can change due to stress. Consider oobleck. If you slowly push your hand through the oobleck, it acts like a liquid. If you pick it up and simply hold it, it will run through your fingers. But if you press it together or apply quick force, it acts like a solid. You can form a ball from it; rip it apart; even run across it. The flow of the oobleck changes with the force you apply, and the more stress you add, the more it acts like a solid instead of a fluid.
There are a few possible theories for why this happens. Cornstarch goop is a suspension fluid- solid cornstarch particles are suspended in liquid water. It could be that sudden friction between the solid cornstarch particles locks them in to make oobleck more solid. Alternatively, the cornstarch solid particles could be pushed closer together and the liquid water becomes trapped; resulting in locked-in clumps of particles and a more solid texture. Scientists from Cornell University and the University of Edinburgh actually did some tests and determined that the friction theory is most likely.
Want to make goop at home? The basic idea is to add enough water to cornstarch to make the desired texture. There isn’t an exact recipe for cornstarch goop. For example, Imagination Station says to use one and a half cups of cornstarch in one cup of water; Babble Dabble Doo uses two cups of cornstarch in one cup of water; and Scientific American says to use 1 to 2 cups of cornstarch in one cup of water. In any case, you start off with your cornstarch in some kind of mixing bowl or pan, and then slowly add in water while mixing until you reach the right consistency. Too watery? Add some more cornstarch. Is it clumping up? Add some more water. You could also mix in food coloring if you don’t like the beige goop, though the food coloring can stain.
Keep calm and science on.
Science On 