Slime Science

The Science of How Slime Works

Slime is a non-Newtonian fluid made by crosslinking molecules to form polymers.
Slime is a non-Newtonian fluid made by crosslinking molecules to form polymers. RubberBall Productions, Getty Images

You know about slime. You've either made it as a science project or else blown the natural version out your nose. Yet, do you know what makes slime different from a regular liquid? Take a look at the science of what slime is, how it forms, and its special properties.

What Is Slime?

Slime flows like a liquid, but unlike familiar liquids (e.g., oil, water), its ability to flow or viscosity is not constant.

So, it's a fluid, but not a regular liquid. Scientists call a material that changes viscosity a non-Newtonian fluid. The technical explanation is that slime is a fluid that changes its ability to resist deformation according to shear or tensile stress. What this means is, when you pour slime or let it ooze through your finger, it has a low viscosity and flows like a thick liquid. When you squeeze non-Newtonian slime, like oobleck, or pound it with your fist, it feels hard, like a wet solid. This is because applying stress squeezes the particles in the slime together, making it hard for them to slide against each other.

Most types of slime are also examples of polymers. Polymers are molecules made by linking together chains of subunits.

Examples of Slime

A natural form of slime is mucous, which consists of mainly of water, the glycoprotein mucin, and salts. Water is the main ingredient in some other types of man-made slime, too.

The classic science project slime recipe mixes together glue, borax, and water. Oobleck is a mixture of starch and water.

Other types of slime are mainly oils rather than water. Examples include Silly Putty and electroactive slime.

How Slime Works

The specifics of how a type of slime works depends on its chemical composition, but the basic explanation is that chemicals are mixed together to form polymers.

The polymers act as a net, with molecules sliding against each other.

For a specific example, consider the chemical reactions that produce classic glue and borax slime:

  1. Two solutions are combined to make classic slime. One is diluted school glue or polyvinyl alcohol in water. The other solution is borax (Na2B4O7.10H2O) in water.
  2. Borax dissolves in water into sodium ions, Na+, and tetraborate ions.
  3. The tetraborate ions react with water to produce the OH- ion and boric acid:
    B4O72-(aq) + 7 H2O <—> 4 H3BO3(aq) + 2 OH-(aq)
  4. Boric acid reacts with water to form borate ions:
    H3BO3(aq) + 2 H2O <— > B(OH)4-(aq) + H3O+(aq)
  5. Hydrogen bonds form between the borate ion and the OH groups of the polyvinyl alcohol molecules from the glue, linking them together to form a new polymer (slime).

The cross-linked polyvinyl alcohol traps a lot of water, so slime is wet. You can adjust the consistency of slime by controlling the ratio of glue to borax. If you have an excess of diluted glue, compared with borax solution, you'll limit the amount of cross-links that can form and get a more fluid slime. You can also adjust the recipe by limiting the amount of water that you use. For example, you could mix the borax solution directly with glue.

This produces a very stiff slime.

Dozens of Slime Recipes To Try