The cornstarch and water mixture known as Oobleck appears to defy the basic scientific rules that govern materials. When gently handled, the substance easily flows and drips, yet a quick, forceful impact causes it to feel completely rigid. This contradictory nature makes it impossible to classify Oobleck simply as either a solid or a liquid, prompting a deeper look into the physics of how materials behave. Oobleck takes us beyond the traditional definitions of matter, showing that a substance’s properties depend entirely on how it is treated.
Identifying Standard States of Matter
Scientists classify materials based on the behavior of their constituent particles. A solid has a fixed shape and a definite volume, retaining its form regardless of the container it occupies. The particles within a solid are tightly packed and only vibrate in place, unable to move freely past one another.
A liquid maintains a definite volume but takes the shape of its container. Liquid particles are more loosely packed than those in a solid, allowing them to roll and slide over each other. This distinction between fixed shape and flow is the traditional boundary line that Oobleck seems to cross.
Oobleck’s Non-Newtonian Nature
Oobleck is classified as a non-Newtonian fluid because its viscosity changes depending on the amount of stress applied. Unlike water, which is a Newtonian fluid with constant viscosity, Oobleck’s internal resistance to flow is variable. When a finger is slowly dipped into Oobleck, it acts like a liquid because low stress allows the particles to move past each other easily.
If the surface is struck hard or squeezed rapidly, the material momentarily resists the force like a solid. This sudden change rejects Newton’s law of viscosity, which describes a linear relationship between applied force and flow rate. Oobleck’s viscosity is dictated by the speed of the interaction rather than an inherent property of the material itself.
The Science of Shear Thickening
The mechanism responsible for Oobleck’s dual identity is called shear thickening. Oobleck is a suspension, consisting of solid cornstarch particles dispersed in water. Under low stress, the water acts as a lubricant, allowing the particles to slide past one another and the suspension to flow easily.
When a sudden, high force, known as shear stress, is applied, the cornstarch particles are instantly pressed together. This rapid compression forces the water out from the narrow gaps between the particles. The temporary removal of the lubricating water causes the particles to jam together, forming transient, rigid structures.
This particle jamming increases the suspension’s viscosity, making the material feel solid for the duration of the force. Once the stress is removed, the water flows back in, the particles separate, and the material instantly returns to its liquid-like state.