A Polymer and a Misbehaving Material–Science for Brownies!

The Magic Bag

Do you think a bag of water will leak if you stab it with a sharp stick??

Materials

Pick Up Kit

  • Resealable plastic bag (thick freeze bags work best)
  • Sharp wooden or bamboo skewers

Materials at home

  • Water
  • A tray or newspaper work surface (optional)

What To Do

  1. Fill the resealable plastic bag with water. You may want to duct tape the top of the bag, especially if working around a computer.
  2. Slowly poke the skewers completely through the bag, in one side, through the liquid and completely through the bag and out the other side. Avoid pushing it through the part of the bag containing air.
  3. See how many skewers you can get in before the bag starts to leak. (it will be a good number!)

What to Talk About

  • Plastic is a polymer, made up of long, elastic molecules that form a seal around the spot where the skewers poke through. This polymer seal prevents the bag from leaking (excessively).
  • Let’s wonder if this activity will work with other fluids.
  • Let’s consider what might happen if we used hot or cold water.
  • Let’s wonder what happens if you poke one end of your stick through the portion of that bag that contains air.
  • If you try some of these things, you may want to do it well away from any computers or electrical stuff.

Cornstarch Goo

Whip up a batch of non-Newtonian Fun!

Materials

Pick up kit

  • 1 Cup + 2 Tbsp cornstarch

Materials at Home

  1. Medium size bowl
  2. 1/2 Cup Water
  3. Spoon (optional)

What To Do

  1. Mix together cornstarch and water in the bowl using a spoon or your fingers. The Goo should be the consistency of thick syrup.
  2. Remove some goo from the bowl and roll it into a ball.
  3. Stop rolling it and let it drip between your fingers.
  4. Put the goo on a tray or cookie sheet. What happens if you slap your hand down on it? Can you make it splash?

What to Talk About

  • Most fluids and solids behave in expected ways and hold their fluid or solid properties when you push, pull, squeeze pour or shake them. However some fluids, known as non-Newtonian fluids, don’t follow the rules!
  • Our cornstarch fluid is called a shear-thickening non-Newtonian fluid and when you apply stress to it, the atoms in the cornstarch rearrange to make it act more like a solid! Ask your Brownies to say “shear-thickening non-Newtonian fluid” six times fast!
  • Observe what the goo does when you handle it. When you let the goo sit in the palm of your hand, it will slowly leak between your fingers and act just like a liquid! But, if you stir it or squeeze it or roll it around in your hands, it looks and feels like a solid!
  • Can you think of any other non-Newtonian fluids? (Ketchup, liquid soaps, and shampoo are all non-Newtonian fluids. These fluids are fairly thick when just sitting there, but become more liquidy as they flow.)
  • Let’s wonder what this kind of a substance could be used for.
  • Let’s wonder what might happen if we added more or less water. Does it retain the same properties?

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