Feb'16

#Tinker: Non-Newtonian Fluid Experiment


by STEM Scouts



This post is an edited version of an experiment write-up originally appearing on Steve Spangler Science.

Watch our experiment on the STEM Scouts Facebook page.

Make your own Non-Newtonian fluid at home using only cornstarch and water. This amazing mixture behaves like a solid and a liquid at the same time. Heads up, this is a messy experiment!

Materials

  • One box of cornstarch (16 oz)
  • Large mixing bowl
  • Cookie sheet, square cake pan, or something similar
  • Water Pitcher
  • Spoon
  • Gallon size zipper-lock bag
  • Newspaper or a plastic drip cloth to cover the floor
  • Water
  • Food coloring

Experiment
Pour approximately 1/4 of the box of cornstarch into the mixing bowl and slowly add about 1/2 cup of water. Stir. Sometimes it is easier to mix the cornstarch and water with your bare hands– of course, this only adds to the fun. Continue adding cornstarch and water in small amounts until you get a mixture that has the consistency of honey. It may take a little work to get the consistency just right, but you will eventually end up mixing one box of cornstarch with roughly 1 to 2 cups of water. Notice that the mixture gets thicker or more viscous as you add more cornstarch.

Pour the mixture onto the cookie sheet or cake pan. Notice its unusual consistency when you pour it into the pan. Stir it around with your finger, first slowly and then as fast as you can. Skim your finger across the top of the glop. What do you notice? Sink your entire hand into the glop and try to grab the fluid and pull it up. Try to roll the fluid between your palms to make a ball.

You can even hold your hand flat on the top of the pan and slap the liquid glop as hard as you can. Most people will run for cover as you get ready to slap the liquid, fearing that it will splash everywhere. Fear not, the mixture stays in the pan. If your mixture inadvertently splatters everywhere, you will know to add more cornstarch.

As you play with the mixture, speculate as to why the liquid behaves in this manner. What causes it to feel like something solid when you squeeze it, yet flow like syrup as it drips off your finger? When you are finished, pour the glop into a large zipper-lock plastic bag for later use.

How Does It Work?
How does the ooze act like a solid sometimes and a liquid at other times?

Actually, the mixture is an example of what is called a Non-Newtonian fluid—a fluid that defies Isaac Newton’s law of viscosity. All fluids have a property known as viscosity. It is the measurable thickness or resistance to the flow of fluid. Honey and ketchup are liquids that have a high resistance to flow.

Newton stated that the viscosity of a fluid can be changed only by altering the fluid’s temperature. For example, motor oil or honey flows more easily when you warm it up and becomes very thick when it gets cold. So, a Non-Newtonian fluid has the same dependence on temperature, but its viscosity can be changed by applying pressure. When you squeeze a handful of glop, its viscosity increases so it acts like a solid for a split second. When you release pressure, the glop behaves just like a liquid.

Ironically, the cornstarch will not stay mixed with the water indefinitely. Over time, the grains of cornstarch will separate from the water and form a solid clump at the bottom of the plastic storage bag. It is for this reason that you must not pour this mixture down the drain. It will clog the pipes and stop up the drain. Pour the mixture into a zipper-lock bag and dispose of it in the garbage.

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