Slime behaves sometimes like a liquid and other times like a solid, appearing to defy simple classification. The transformation from common household glue and an activator solution into a stretchy, cohesive mass is a result of fundamental principles in polymer chemistry. Understanding the science of slime provides a tangible way to explore the molecular interactions that govern the properties of many everyday materials.
The Building Blocks: Polymer Chains
The foundation of most homemade slime is glue containing the polymer polyvinyl alcohol (PVA). Polymers are macromolecules made up of long, repeating chains of smaller molecular units. In its liquid state, the PVA component consists of countless individual polymer chains suspended in water.
These long, flexible molecules are tangled but not chemically bonded, allowing them to easily slide past one another. This movement is why the glue flows freely and maintains a liquid consistency. The viscosity of the starting glue solution is a direct result of these chains moving and interacting, but lacking any permanent structural connection. When the activator is introduced, the goal is to chemically alter this arrangement.
Cross-Linking: The Chemical Reaction
The transformation from liquid glue to viscoelastic slime occurs through cross-linking, which chemically connects the separate polymer chains. This reaction is initiated by the activating ingredient, typically a source of borate ions released when compounds like sodium tetraborate dissolve in water. The borate ion acts as a temporary molecular bridge between the long PVA strands.
PVA molecules contain hydroxyl (\(text{OH}\)) groups that are attractive to the borate ions. The borate ion forms weak, non-covalent bonds with the hydroxyl groups on two different PVA chains simultaneously. This action links the strands together, creating a three-dimensional network or gel.
Because these connections are numerous but weak, they allow the structure to stretch and deform without breaking. The resulting viscoelastic polymer network traps water, giving slime its cohesive texture. The final texture is directly proportional to the number of cross-links formed.
Why Slime is a Non-Newtonian Fluid
The cross-linked network exhibits the properties of a non-Newtonian fluid. A standard, or Newtonian, fluid like water maintains a constant viscosity regardless of the force applied. In contrast, a non-Newtonian fluid’s viscosity changes based on the shear stress, or force, exerted upon it.
Slime demonstrates shear-thickening behavior, meaning it becomes thicker and more resistant to flow when forced quickly. When handled gently, the molecular cross-links temporarily separate and re-form, allowing the polymer chains to slide slowly past one another, making the material stretch and flow like a viscous liquid. However, applying a sudden, sharp force causes the polymer chains to jam together instantaneously. This momentary resistance to flow makes the slime act like a solid before it relaxes back into its fluid state.
Common Activators and Their Chemistry
Common activators used to make slime all serve the same chemical purpose: supplying the borate ions necessary for the cross-linking reaction. The three primary methods for activation are:
- A dissolved solution of Borax powder, which is sodium tetraborate.
- Contact lens solution combined with baking soda. The solution must contain boric acid, and the baking soda (sodium bicarbonate) increases the \(text{pH}\) to convert the boric acid into active borate ions.
- Liquid laundry starch, which contains sodium tetraborate that releases the required borate ions upon mixing with the glue.