Slime is a source of common confusion because the substance appears to be both a liquid and a solid. When left alone, it relaxes and flows like a liquid, conforming to the shape of its container. Yet, if struck or squeezed quickly, it resists the force and feels firm, behaving like a solid. This dual nature means that slime defies simple classification into the traditional states of matter.
The Ingredients That Create Ambiguity
Slime is the result of a precise chemical reaction between two primary components: a polymer base and a cross-linker. The polymer base is typically a solution of polyvinyl alcohol (PVA), which consists of long, chain-like molecules suspended in water. These PVA chains are initially able to slide past one another freely, making the solution a simple liquid.
The transformation occurs with the introduction of the second component, a solution containing borate ions. These borate ions act as temporary bridges, chemically linking the long PVA chains together at various points. This process, called cross-linking, creates a three-dimensional, loosely connected network of polymer strands.
What Defines a Non-Newtonian Fluid
Understanding slime requires exploring the concept of viscosity, which describes a fluid’s resistance to flow. For many common fluids, like water or oil, the viscosity remains constant regardless of how much force or “shear stress” is applied to them. These are known as Newtonian fluids, and their flow rate is directly proportional to the force exerted.
Slime, however, belongs to a group of substances called non-Newtonian fluids, where the viscosity is variable and changes depending on the applied shear rate. The fluid’s resistance to flow is directly tied to how quickly it is being deformed.
The material’s internal structure, composed of those long polymer chains, dictates this non-linear relationship between stress and flow. When force is applied, these microstructures align, deform, or jam together, which instantly alters the fluid’s internal friction.
How Slime Exhibits Liquid and Solid Properties
Slime is specifically classified as a shear-thickening non-Newtonian fluid, meaning its viscosity increases when the shear rate is high. The loose network of polymer chains created by the borate cross-linkers explains this behavior.
When the material is handled gently, the applied force is low and the rate of deformation is slow. Under this low shear rate, the temporary cross-links between the PVA chains have sufficient time to break and re-form, allowing the polymer network to reorganize and the substance to flow. This slow, liquid-like movement permits the slime to stretch and pour from its container, conforming to the shape of the vessel.
Conversely, if a high shear rate is applied—such as punching the material, rolling it into a ball, or pulling it sharply—the polymer chains do not have enough time to separate and link back together. The sudden, intense force causes the long strands to rapidly tangle and jam against each other. This instantaneous resistance to flow increases the viscosity dramatically, causing the slime to behave rigidly and resist the force like a temporary solid.
Other Examples of Stress-Dependent Substances
Slime is not unique in its ability to change its resistance to flow, as many other common materials exhibit non-Newtonian properties. These substances are broadly categorized by how their viscosity changes with stress. Slime is a shear-thickening material, where increased stress leads to increased viscosity.
Another common example of a shear-thickening fluid is Oobleck, a simple suspension of cornstarch and water, which instantly solidifies when struck. In contrast, many everyday liquids are shear-thinning, meaning their viscosity decreases with increased shear rate. Ketchup, for instance, seems thick until it is shaken or squeezed, at which point the internal structure aligns and allows it to flow easily.