Oobleck, a simple mixture of cornstarch and water, challenges our everyday understanding of matter. When left alone, this slurry flows easily like a liquid, yet a quick, sharp impact causes it to feel instantly solid. This confounding behavior is why Oobleck is a fixture in science demonstrations. To understand this mixture, we must first establish the difference between physical and chemical transformations.
Defining Changes in Matter
Changes in matter are broadly categorized as either physical or chemical, based on whether the substance’s molecular identity is altered. A physical change involves a substance changing its form or state without changing its fundamental chemical composition. Examples include melting ice into water or tearing paper, where the molecules remain chemically the same throughout the process.
Chemical changes, in contrast, result in the formation of entirely new substances with different properties from the starting materials. This transformation occurs when chemical bonds are broken and new bonds are formed, leading to a new molecular structure. Evidence of a chemical change often includes the sustained production of heat or light, the release of a gas, or an irreversible change in color.
The Unique Behavior of Oobleck
Oobleck is a prime example of a non-Newtonian fluid, a class of substances whose viscosity changes in response to applied stress. When the mixture is gently handled or allowed to flow slowly, it behaves just like a typical liquid, easily conforming to the shape of its container. A hand can sink into it slowly, and it will drip off the fingers if poured.
However, the application of sudden, intense force, known as shear stress, instantly causes the mixture to resist flow. If one punches the surface, it will momentarily feel firm and solid. This temporary solidification is a macroscopic manifestation of its unique rheological properties. Unlike water, Oobleck dramatically increases its viscosity when stressed.
The Verdict: A Physical Transformation
The dual nature of Oobleck is the result of a physical change, not a chemical one. When cornstarch and water are mixed, no new chemical compound is created; the molecules retain their original chemical structures. The mixture is classified as a colloid, specifically a suspension, where tiny solid particles are dispersed but not chemically bonded within a liquid medium.
The temporary solid-like state is fully reversible, which is a hallmark of a physical change. Once the external force is removed, the mixture immediately returns to its liquid-like state without permanent alteration. There is no evidence of chemical reaction, such as heat generation, gas evolution, or an irreversible change in composition, that would signal a chemical transformation. The change observed is solely in the physical arrangement and interaction of the dispersed particles.
The Science Behind Shear Thickening
The physical mechanism responsible for Oobleck’s unique behavior is called shear thickening. This phenomenon is rooted in the structure of the cornstarch granules, which are microscopic, irregular particles suspended throughout the water. The water acts as a lubricating medium, allowing these particles to slide past one another easily when the mixture is at rest or under low stress.
When a sudden, high-intensity force is applied, the water is unable to flow quickly enough to fill the space between the rapidly moving particles. This movement effectively squeezes the water out from the narrow gaps separating the granules. The lack of lubricating water forces the cornstarch particles into direct, frictional contact, a process known as particle jamming.
This frictional jamming creates a temporary network of solid-like resistance that spans the material under stress. The material’s viscosity increases dramatically, causing it to feel solid and able to support weight. When the external stress is released, the water quickly flows back into the gaps, restoring lubrication and allowing the cornstarch granules to slide freely once more.
The shift is a type of reversible phase transition known as a re-entrant solid transition, where the application of force causes the suspension to move from a liquid-like state to a jammed, solid-like state. This entire process is a physical rearrangement of existing components, fundamentally different from a chemical reaction. This physical mechanism allows Oobleck to be used in advanced materials research, such as developing impact-resistant body armor or flexible speed bumps that stiffen only when struck.