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!
Background
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.
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 stir 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 leave 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.