Slime Science: The Strange Chemistry Behind Stretch, Squish & Bounce
Slime is a soft, highly deformable material that behaves somewhere between a liquid and a solid. Its unusual properties come from a polymer network: long molecules become lightly linked together, allowing the material to stretch and flow while still holding its shape.
Properties of slime
Typical slime has several interesting characteristics:
- Viscoelasticity: It behaves partly like an elastic solid and partly like a viscous liquid. Pull it slowly and it stretches; pull it rapidly and it may snap.
- Non-Newtonian behavior: Its apparent viscosity changes with how it is handled. Vigorous stirring or squeezing can make it behave differently from simply letting it sit.
- Stretchiness: Long polymer molecules can slide past one another while remaining connected, allowing considerable deformation.
- Cohesion: The polymer network holds the material together rather than allowing it to separate like ordinary water.
- Adhesion: Slime can stick to skin, glass, plastic, and other surfaces, depending on its formulation.
- Shear thinning: Many slimes become easier to flow when subjected to sustained shearing, such as stirring or kneading.
- Elastic recovery: After being stretched or deformed, slime can partially return toward its original shape.
- Water content: Most common slimes contain a large amount of water. As the water evaporates, the slime becomes progressively firmer and eventually dries out.
- Temperature dependence: Warm slime generally flows more readily, while cold slime tends to become stiffer.
The basic chemistry
One of the simplest ways to make slime uses polyvinyl alcohol (PVA) and a borate compound.
PVA is a polymer—a very long molecule made from repeating units. When borate ions interact with appropriate groups along the PVA chains, they form temporary links between the chains:
PVA chain — borate link — PVA chain
With enough links, the individual polymer molecules become a loose three-dimensional network. Water remains trapped within that network, producing the characteristic stretchy gel we recognize as slime.
This is why slime is quite different from simply mixing glue and water: the important step is cross-linking the polymer chains.
A simple recipe
A common classroom formulation can be made from washable PVA-based school glue and a dilute borate solution.
Materials
- PVA-based white school glue
- Water
- Baking soda
- Contact-lens solution containing boric acid and/or sodium borate
- Mixing bowl
- Spoon or craft stick
- Optional food coloring
Construction
- Put about ½ cup of PVA school glue into a bowl.
- Add about ½ cup of water and mix thoroughly.
- Add a few drops of food coloring if desired.
- Mix in about ½ teaspoon of baking soda.
- Add contact-lens solution a little at a time, stirring continuously.
- As cross-linking begins, the mixture will stop behaving like ordinary liquid glue and start forming a cohesive mass.
- Remove it from the bowl and knead it with your hands for several minutes.
- If it remains excessively sticky, add a very small additional amount of contact-lens solution and knead again.
The exact amounts aren't critical because different glues and contact solutions have different polymer concentrations and borate contents.
Changing the properties
The formulation can be deliberately adjusted.
| Change | Result |
|---|---|
| More cross-linking solution | Firmer, less sticky slime |
| Less cross-linking solution | Softer, stickier, more fluid slime |
| More water | Looser slime |
| More PVA | Generally thicker and more substantial slime |
| Kneading | More uniform texture |
| Adding shaving foam | Light, fluffy “fluffy slime” |
| Adding small amounts of starch | Different, often thicker texture |
| Adding glitter/pigment | Changes appearance without fundamentally changing the polymer network |
There is an important distinction between slime and oobleck. Oobleck, made from cornstarch and water, is a suspension of solid particles and does not rely on a cross-linked polymer network. Slime, by contrast, is primarily a polymer gel.
Safety
Don't eat slime, and wash hands after handling it. Avoid getting it in the eyes, and keep it away from young children and pets who might ingest it. Contact-lens solution should be used only according to its label; the recipe specifically relies on its borate-containing ingredients.
