Oobleck, a mixture of cornstarch and water, exhibits unusual properties. When left undisturbed, it flows like a liquid, yet it can transform into a firm, resistant material under sudden impact. This dual nature makes Oobleck a popular scientific demonstration.
Oobleck’s Unique Actions
If you gently dip your hand into a bowl of Oobleck or pour it, it readily flows through your fingers, behaving much like a thick liquid. It will readily take the shape of its container, much like water.
However, when subjected to a sudden, strong force, such as punching Oobleck, hitting it with a hammer, or even running across a pool of it, it causes it to momentarily stiffen and act like a solid. As soon as the force is removed, it quickly returns to its liquid-like state, flowing freely once again.
Understanding Non-Newtonian Fluids
Oobleck is a non-Newtonian fluid. Unlike Newtonian fluids, such as water or oil, whose viscosity remains constant, non-Newtonian fluids exhibit a variable viscosity. Their “thickness” or resistance to flow changes in response to stress or shear rate.
This means that for a non-Newtonian fluid, the relationship between applied force and its flow behavior is not linear. Oobleck specifically demonstrates shear-thickening behavior, where its viscosity increases when subjected to higher shear rates or sudden forces. Other non-Newtonian fluids, like ketchup, are shear-thinning, becoming runnier when shaken.
The Mechanism of Shear Thickening
The unique behavior of Oobleck stems from its composition as a concentrated suspension of cornstarch particles within water. Cornstarch particles do not dissolve in water; instead, they remain suspended, forming a dense mixture. Under low stress, the water acts as a lubricant, allowing these particles to slide past one another, enabling the mixture to flow like a liquid.
When a sudden, strong force is applied, the water is rapidly forced out from between the cornstarch particles. This action causes the particles to pack together very tightly, increasing the friction between them. The particles effectively jam or lock into position, forming temporary clusters that resist further compression and give the Oobleck a solid-like rigidity. This phenomenon, sometimes explained by a “hydro-clustering” mechanism, involves particles being driven into clusters by hydrodynamic lubrication forces overcoming repulsive forces. As soon as the external force is removed, water flows back into the spaces between the particles, allowing them to separate and the Oobleck to become fluid again.
Everyday Examples
Shear thickening, as seen in Oobleck, has various practical applications. Shear-thickening fluids are being explored for use in protective gear, such as liquid body armor or impact-resistant clothing. These materials can remain flexible during normal movement but instantly stiffen upon impact, absorbing energy and protecting the wearer.
Another potential application involves their use in speed bumps that could become firm only when a vehicle passes over them at high speed, or in specialized dampers and shock absorbers. While quicksand is often cited, it actually behaves more like a shear-thinning fluid when disturbed.