Oobleck’s behavior often challenges our understanding of how materials exist. This substance can feel solid one moment and flow like a liquid the next, prompting a fundamental question: Is Oobleck a solid or a liquid? This article explores Oobleck’s unique properties and its relation to a broader scientific concept.
What Exactly is Oobleck?
Oobleck is a simple mixture of cornstarch and water, typically 1.5 to 2 parts cornstarch to 1 part water. Its name originates from the Dr. Seuss book, “Bartholomew and the Oobleck.” When left undisturbed, Oobleck appears to be a thick liquid, effortlessly pouring and taking the shape of its container.
However, its properties change dramatically when force is applied. If you quickly hit, squeeze, or punch Oobleck, it resists the pressure and feels firm, almost like a solid. Yet, if you relax your grip or slowly dip your hand into it, the mixture flows easily through your fingers.
The Non-Newtonian Secret
Oobleck is neither a true solid nor a true liquid; it is a type of non-Newtonian fluid. Unlike common Newtonian fluids such as water, whose viscosity remains constant, a non-Newtonian fluid’s viscosity changes with stress or force.
Oobleck specifically falls into the category of a shear-thickening fluid, also known as a dilatant fluid. In these fluids, applying a rapid shear force causes the viscosity to increase, making the substance thicker and more resistant to flow. When a fast force is applied, the cornstarch particles within the water are forced closer together, causing them to lock up and trap the water, forming a temporary solid-like structure. This jamming increases internal friction and resistance, making Oobleck feel solid when struck.
When the pressure is released or a slow force is applied, the particles can once again roll over each other, allowing the cornstarch to flow freely. The water acts as a lubricant between the cornstarch granules when the force is gentle, allowing them to move freely. This behavior allows someone to even run across a large pool of Oobleck without sinking, as long as they apply enough force with each step to thicken the fluid beneath their feet.
Where Else Do We See This?
Non-Newtonian fluids are found in many everyday substances and specialized applications. Some non-Newtonian fluids exhibit shear-thinning properties, meaning their viscosity decreases with increased shear force. Ketchup is a common example; it is thick and difficult to pour when still, but becomes runnier and flows easily when shaken or squeezed.
Toothpaste is another familiar non-Newtonian fluid, specifically a Bingham plastic, which requires a certain amount of force to start flowing. It maintains its shape on a toothbrush but flows readily when squeezed from the tube. Quicksand is also a shear-thinning non-Newtonian fluid. When agitated, its viscosity decreases, which is why struggling in quicksand can cause one to sink deeper.
Non-Newtonian fluids also have practical applications in various industries. Shear-thickening fluids are being researched for use in flexible body armor, which could offer protection by hardening upon impact. In the food industry, these fluids are used to control the texture and flow of products like sauces, dressings, and ice cream. They are also important in cosmetics, pharmaceuticals, and industrial processes like drilling.