When cornstarch, a common household ingredient, is combined with water, it forms a substance that behaves in an unexpected manner. This simple mixture acts like both a liquid and a solid, depending on how it is handled. This dual nature makes it a compelling subject for everyday science explorations, prompting questions about its classification and the underlying scientific principles governing its unusual behavior.
The Common Name and Scientific Classification
This mixture of cornstarch and water is widely recognized by the name “oobleck.” The name originates from a Dr. Seuss book, “Bartholomew and the Oobleck,” where a gooey green substance falls from the sky. Scientifically, oobleck is classified as a non-Newtonian fluid. Unlike Newtonian fluids, such as water, whose viscosity remains constant regardless of the force applied, a non-Newtonian fluid’s viscosity changes under stress.
Oobleck specifically falls into the category of a “shear-thickening” non-Newtonian fluid. This means its viscosity, or resistance to flow, increases when a force, or shear, is applied. Conversely, when the force is removed, it reverts to a more liquid state. This property contrasts with shear-thinning fluids, like ketchup, which become runnier when shaken or stressed.
Understanding Its Unique Behavior
The unique behavior of oobleck stems from the interaction of its two components: cornstarch particles suspended in water. Cornstarch consists of tiny, solid starch particles, which do not dissolve in water but remain dispersed throughout it. When the mixture is at rest or subjected to slow movement, the water acts as a lubricant, allowing the starch particles to slide past each other easily, giving it a liquid-like consistency.
However, when a sudden, strong force is applied, such as a punch or a quick squeeze, the cornstarch particles are forced closer together. This rapid compression causes the particles to jam or lock into a temporary, rigid structure. The water trapped between these tightly packed particles forms a semi-rigid structure, making the mixture feel solid. This phenomenon is known as shear thickening, where the material’s resistance to flow increases with increased shear rate. When the applied stress is removed, the particles separate, and the mixture flows like a liquid once more.
Practical Applications and Demonstrations
The unique properties of cornstarch and water make it a popular subject for home science experiments. People often observe its dual nature by slowly dipping a hand into it, which allows it to flow, but then attempting to pull it out quickly, which feels as though it resists the motion. Punching or slapping the surface demonstrates how it momentarily becomes solid, preventing splashing. Another common demonstration involves rolling a ball of oobleck in one’s hands; as long as continuous pressure is applied, it remains solid, but it quickly melts back into a liquid when the pressure is released.
Beyond simple demonstrations, the principles of shear-thickening fluids are being explored for various real-world applications. These include potential uses in protective gear, such as body armor and helmets, where the material could remain flexible under normal conditions but stiffen upon impact to absorb energy. Other potential applications include vibration damping, shock absorption in mechanical systems, and in some food products where controlled viscosity is beneficial.