by Melissa Jaramillo and Julie Snyder
Generally we classify things as liquid or solid or gaseous but there are some substances that don't behave like expected. Have you ever played with Silly Putty®, Ooze or Slime? (Or rather played along with the kids...surely parents don't "play.") You can slowly stretch silly putty out into a smooth, thin circle. Hit it with a hammer and it shatters! Is it a liquid because it flows? Or is it a solid because it is brittle? These materials belong to a special class of matter that doesn't obey the usual laws of viscosity.
Viscosity is a property of liquids that describes resistance to flow. Pour about a ½ cup of water into a glass. Try sloshing it around and pouring it out. See how easily it moves? Water has low viscosity because it flows quickly. Now pour about ½ molasses into a glass. Does it move as quickly and easily as water? It seems a lot more sluggish, doesn't it? Molasses has a higher viscosity because it flows much more slowly. Most fluids are Newtonian -- their ability to be poured depends on temperature. The warmer molasses is, the faster it will flow. Stick it in the microwave for about 30 seconds and pour it again. Is it faster or slower this time?
Non-Newtonian fluids like Silly Putty® and Slime depend on a different factor than temperature to determine their ability to be poured. A low stress such as pouring or pulling slowly allows non-Newtonian fluids to flow and stretch. A high stress such as hitting firmly or pulling sharply will cause the material to break.
Each non-Newtonian fluid is made of polymers. These polymers are molecules with long chains of repeating units. Take a box of paper clips and build several 18-inch long chains. Hold one end of your chains with one hand. Place two or more chains between fingers and slowly run your hand down the chains. You are demonstrating a low stress. The polymers (in this case, paper clip chains) can act like a liquid and slide past each other if stress is low. It is rather like running your fingers slowly through your hair. Your hand can move smoothly from the top to the bottom of the strands of hair.
If stress is high, like jerking a comb through tangles, the polymers tend to catch on each other and behave like solids. Try a hard, fast motion with the paper clips chains. Hold one end the chains in one hand. Wrap your other hand snuggly around all the chain. Move quickly toward the ends. Notice how some of the clips turn sideways and snag on other clips. Your "polymer" is now acting like a solid.
Below are four simple recipes for non-Newtonian fluids. Make one or all four as an introduction to these special fluids.
If desired, add food coloring to water before mixing. Add water slowly, using fingers to mix until the mixture is gooey and fluid like. It has the right consistency if it will drip slowly from your hand and doesn't splatter when hit firmly.
• 1 tablespoon glue
• 1 tablespoon water
• 2 teaspoons borax solution*
• popcicle stirring stick
*Mix 2 tablespoons borax in 2 cups water. Add a few drop lysol cleanser to retard mold growth.
• 2 tablespoons 4% polyvinyl alcohol solution (PVA)
• 1-2 drops food coloring (optional)
• 1/2 teaspoon borax solution (above)
• popcicle stirring stick
Pour PVA in cup. Add food coloring and stir. Continue stirring while adding the borax solution. Once the gel has formed, remove it from the cup and knead in your hands. Store in a zipper bag.