Goal: Scouts will understand that slime is a crosslinked polymer and the properties of a non-Newtonian fluid.
Supplies: Elmer's clear glue (diluted 50:50 with water), powdered Borax for cross linking (find it at the grocery store in the laundry aisle), measuring spoons, plastic cups, water, food coloring*, small plastic “snack” bags. (*These items are in the Color Your World with Polymer Science kit found at the scout store.)
When the scouts are “stirring” the slime ask them if it feels cool or warm. (cool) Discuss that this is an example of an endothermic reaction—one that takes ‘in’ heat. An endothermic reaction absorbs energy (heat) instead of giving off energy (heat). An endothermic reaction feels cool because the reaction pulls heat from your finger into the slime. An exothermic reaction would feel warm/hot to the touch as it gives off heat.
What is a non-Newtonian fluid? Pressure-dependent substances, like slime (and silly putty and quicksand) are non-Newtonian fluids. In a non-Newtonian fluid, viscosity changes depending on the force applied and it behaves either more like a liquid or more like a solid.
Often a substance changes its state because of a change of temperature—like freezing water to make a solid ice cube or boiling water to make steam which is a gas. But this simple mixture shows how changes in force can also change the properties of some substances.
The slime will behave differently depending on the amount of force applied. Let it sit on a surface or in a cup or bag and observe how it slowly moves like a super thick fluid. Roll the slime into a ball and drop it from about twenty (20) inches on a hard surface and observe how it behaves like a bouncy solid.
When pressure is applied to a non-Newtonian fluid (like when you squeeze) it increases the resistance to the force (or viscosity). A fast tap on the top of the slime and it feels hard, but if you dip your finger in slowly the mixture doesn’t resist the force and behaves more like a fluid.
Explain force to younger scouts with this activity: Have the scouts each make a small sign that says BIG and another sign that says LITTLE. Then play “Big or Little Force?” with scouts. The troop leader asks if slamming a door is a Big or Little force? What about quietly closing a door? Are hiccups Big or Little? A galloping horse? An explosion? Riding a roller coaster? Rolling a marble or throwing a ball? To answer, the girls hold up the appropriate sign. Have scouts think of their own BIG and LITTLE force examples.
Outcome: Students will understand that forces can be big or little and slime behaves differently depending on the force applied because it is a non-Newtonian fluid.