A rabbit is fluffy, mud is squishy, and a balloon is stretchy. What substances can be fluffy, squishy and stretchy at the same time, and are so much fun to play with? Silly Putty, Gak and slime!
These substances can be confusing, too. Most substances become harder when cooled and flow much better as they warm up. Think of how honey slowly oozes from the bottle on a cold day and rushes out on a hot day. Silly Putty, Gak and slime are different. They can feel as hard as a solid when squeezed in your fist, but as soon as you release your grip, they ooze out through your fingers like a thick liquid. Why would slime be different? In this activity you will make your own slime, play with it and discover what makes it flow!
Is it a solid or a liquid? Solids consist of tightly packed particles called molecules or atoms that clasp onto each other so the solid holds its shape. Liquids have particles that can slide over and around one another, allowing the fluid to flow. Only adding or taking away heat can make some liquids, like water or oil, flow better or worse. These are called Newtonian liquids. Non-Newtonian liquids, such as ketchup and slime, are different. Manipulations like squeezing, stirring or agitating can also change how they flow. Sometimes they can become so viscous—or have such a hard time flowing—that they could easily be mistaken for a solid.
One such non-Newtonian liquid can be created with white school glue, which is a polymer. A polymer is made from long chains of repeating parts called monomers. One polymer might consist of hundreds of thousands of monomers. Polymers are also called macromolecules, or large-sized molecules. Some are man-made, such as plastic and nylon. Others occur in nature, such as DNA, wheat gluten and starches.
White school glue is liquid because its long polymers can slide over and along one another. It does not flow easily, though; it is quite viscous. The addition of some chemicals—such as a borax solution (or sodium tetraborate decahydrate dissolved in water)—can cause cross-links to form between the polymers. It is as if the very long molecules started to hold hands. Will the result still be a fluid where the polymers can glide over each other, or will it become a solid?
- Elmer's glue or other polyvinyl acetate (PVA) glue
- Hot water
- Stirring rod or plastic spoon
- Measuring spoon (one half tablespoon)
- Measuring cups (one half and one quarter cups)
- Goggles or eye protection (handling borax can irritate eyes)
- A work space (and work clothing) that is protected and won't be damaged if sticky slime gets on it
- Adult helper
- Food coloring or marker (optional)
- Ziplock bag or airtight container to store your slime (optional)
- Protect your work space and clothing—slime can be sticky and hard to remove!
- Put on goggles or glasses, as the borax solution can irritate the eyes.
- Have an adult helper stir one half tablespoon of borax powder into one half cup of lukewarm water in a cup. Stir well until the solution looks clear, label the container "2 percent borax solution," and set aside.
- Pour one half cup of glue and one quarter cup of warm water in a container. Note that this solution has 2/3 (or 67 percent) glue.
- Optional: If you like colored slime, you can mix in a few drops of food coloring. Another option is to put the tip of a marker into the water for a short time so the ink dissolves in the water.
- Stir the glue/water mixture with the stirring rod.
- Add five tablespoons of the borax solution to your glue/water mix.
- Stir with the stirring rod. After some stirring, you should see a substance sticking to your stirrer. Does the sticky substance look like a solid or like a liquid—or can you not tell yet?
- If your substance is still watery, add more borax solution ¼ tablespoon at a time until there is very little watery solution left.
- Collect the sticky substance in your hands and work it with your hands for about one minute. How does the slime feel? How does the stickiness and stretchiness change when you work it for a while?
- Would you say the slime is a liquid, or is it more like a solid?
- Squeeze your slime into an oval and use both hands to pull it apart quickly. Does it tear or elongate?
- Squeeze your slime into an oval again and use both hands to pull it apart slowly. Does it tear or elongate? How thin can you get it?
- Work your slime with your hands to form a ball. Try to stick your finger into it forcefully. How deep does your finger go? Does it feel like you poked your finger into something solid, or something liquid?
- Now try to stick your finger into it gently. How deep does your finger go? Does it feel like you poked your finger into something solid, or something liquid?
- Squeeze your slime into a ball again and put it in a container. What do you think will happen if you leave it there for a while? Will it stay in an oval, like a solid would do, or will it relax into a puddle and take the form of the container, like a liquid would do?
- Optional: To keep your slime nice and soft, store it in an airtight container or ziplock bag.
- Extra: Add other substances, such as shaving cream or liquid soap, to your glue/water solution. Will you still obtain slime? How will this slime feel and look different?
- Extra: Leave your slime uncovered for a day. What do you think will happen? Will it become more like a solid, or start to flow easier? Why do you think this will happen?
Observations and results
Did the slime sometimes feel like a solid and sometimes like a fluid? This is expected.
This type of slime thickens or becomes harder or more viscous when you squeeze or stir it. This happens because it is made up of very long particles that are cross-linked. When you leave the particles alone they will coil up, and the coils can slide over each other. When you apply pressure by squeezing or stirring, some coils unwind and become entangled, making it harder for the slime to flow.
When you stirred your slime, tried to rip it apart or poked your finger into it with force, the polymers were entangled and it looked like a solid. As a result, it was hard to stir, it ripped apart, and your finger bounced back.
When you left your slime alone to rest, gently pulled it apart or gently poked your finger in it, the polymers were curled up. They could slide over one another, and it felt more like a liquid. As a result, the slime took the form of the container, it could be stretched thin, and your finger could move through it. It did not flow as easily as water because it consists of long cross-linked particles, whereas water consists of small particles. When a substance keeps its volume but loses its form when left alone, scientists call it a liquid.
Do not pour glue solutions or slime down a drain because they can form clogs. Instead, throw them away in the garbage. Wash all equipment with soapy water.
More to explore
It's a Solid... It's a Liquid... It's Oobleck!, from Scientific American
The Scientific Secret of Stretchy Dough, from Scientific American
Playing with Polymers, from Scientific American
This activity brought to you in partnership with Science Buddies.