When cornstarch and water are mixed, the result is a substance that appears to defy the typical rules of physics, behaving like a liquid one moment and a solid the next. This peculiar mixture is a favorite in kitchen science experiments because of its surprising dual nature. It can be effortlessly poured like water, yet it instantly resists force when struck or squeezed.
Identifying the Unique Mixture
The substance created by combining cornstarch and water is officially known as a colloidal suspension, though it is popularly called Oobleck, a name borrowed from a Dr. Seuss story. A suspension is a heterogeneous mixture where tiny solid particles are dispersed throughout a liquid medium but are not chemically dissolved. In this specific case, the microscopic cornstarch granules are suspended in the water, forming a cloudy, opaque fluid. Because the particles remain separate instead of integrating into a true solution, the cornstarch will eventually settle out at the bottom if the mixture is left undisturbed.
The Science Behind Non-Newtonian Behavior
The unusual characteristic of the cornstarch and water mixture is that it is a non-Newtonian fluid, meaning its viscosity, or resistance to flow, is not constant. Unlike a Newtonian fluid, such as water, whose viscosity only changes with temperature, the viscosity of this mixture changes based on the amount of force, or shear, applied to it. Specifically, the cornstarch mixture is a shear-thickening fluid, which means it becomes dramatically thicker when subjected to a sudden force.
This phenomenon occurs because of the physical interaction between the suspended cornstarch particles when pressure is applied. Under slow movement, the water acts as a lubricant, allowing the particles to slide past each other easily, so the substance flows like a liquid.
However, when a sudden, high-force impact occurs, the water is quickly forced out from the gaps between the cornstarch granules. This rapid displacement causes the particles to momentarily jam or lock together, creating a temporary, solid-like structure that resists the force. The fast impact traps the water between the rough surfaces of the cornstarch molecules, which creates a semi-rigid mass.
Once the pressure is removed, the water flows back in, the particles unjam, and the substance returns to its fluid state. This transition from liquid to solid and back again is what allows a person to walk or run quickly across a large vat of the mixture without sinking, as long as they maintain a continuous, high-speed impact with each step.
Practical Steps for Creating the Mixture
Making this unique substance at home is simple, requiring only cornstarch and water. The ideal ratio for a robust, shear-thickening mixture is generally between one part water to 1.5–2 parts cornstarch. Start with the water and gradually mix in the cornstarch to achieve a consistency that feels like a thick, loose liquid when stirred slowly. Adjust the mixture by adding more cornstarch if watery, or water if stiff. To test the result, slowly dip a finger into the substance, which should sink easily, but a quick punch should cause the material to feel hard and resist the impact.