Bathtub Science at Home
The Science behind the mess….
How does it work?
Cornstarch and water (affectionately known as “Oobleck” after Dr. Seuss) is a mixture of tiny cornstarch particles suspended in water that together form what is known as a “jamming material”. These materials can have very different physical behavior depending on how fast you try to deform it. If you deform the material slowly, the cornstarch particles are lubricated by the water and slide past each other allowing the material to flow like a fluid. However, if you try to deform it quickly, the particles jam up against each other forming an interlocked network of particles that act like a solid. If you push on the cornstarch water mixture fast enough you can even get it to fracture. Quicksand is another example of a jamming material.
What is Rheology?
The study of how materials behave when you try to deform them is known as the study of “Rheology”. Cornstarch/water mixtures have very complicated rheology that depends on the rate of deformation. Deformed slowly over long periods of time, Oobleck behaves like a fluid and flows. Deformed quickly over short periods of time, Oobleck behaves like a solid.
What this has to do with the Earth…
The important idea of this demonstration (beyond having fun and making a huge mess), is that rocks in the Earth have very similar rheology to Oobleck, but over much longer time scales. If you try to deform a rock quickly it can fracture and break, and this ability to act as a “brittle” and “elastic” solid, gives rise to earthquakes. However, on time-scales of millions of years, at high temperatures and pressures, solid rocks can actually flow like toothpaste. The ability for rocks to behave both as a solid and fluid allows for mantle convection and plate tectonics, which are responsible for most of the exciting features of the solid-earth like mountain building, earthquakes, and volcanoes. Without this ability, the planet would be a rather dull place to live.