Water and cornstarch make a non-Newtonian fluid when mixed: messy but great fun!Sixth grade was a big year for science fair projects in my hometown. I was fascinated by sound and decided to test whether high or low pitches traveled more easily. In principle this could have been a great idea, but I soon discovered that having family members lie down on the living room floor trying to listen while a beige plastic Fisher Price cassette player honked various toots and tweets from the adjacent room just wasn’t going to work out as well as I had hoped.
Fast-forward fifteen years to the beginning of the present school year and the Internet has given us all a huge leg-up in finding hands-on ways to learn science. These are demonstrations rather than experiments–an important difference for those entering a fair. Nevertheless, I have included two of my favorites below.
Pay tribute to Dr. Seuss’s book Bartholomew and the Oobleck by whipping up this mixture that is both solid and liquid at the same time! The simplest version is listed below, but adding a few more bells and whistles can increase the demonstration’s awe-factor a bunch.
What to do: You need a mixing bowl, water, and cornstarch. Fill the mixing bowl with about 1 cup of cornstarch, and add roughly an equal volume of water. Mix, incrementally adding cornstarch or water until the mixture attains an appropriate blend of goopiness and firmness. Enjoy the fluid’s bizarre properties by squishing and kneading it with your hands.
What’s going on? Nearly all fluids have some intrinsic flow resistance. This property, called viscosity, is the reason water flows more easily than honey and at least partly why Usain Bolt can run 100 meters in under 10 seconds while it takes Michael Phelps well over a minute to swim the same distance. Our water/cornstarch mixture has a very special viscosity, making it easy to dip your hand into the mixture slowly, but quite hard to push it in quickly. (Technically, this is an example of a non-Newtonian fluid.) Science class will teach you that almost all matter can be classified into either a solid, liquid, or gas, but this is at least one example where the distinctions blur.
Bernoulli’s Hair Dryer:
In 1738 the mathematician Daniel Bernoulli (pronounced Ber-NEW-lee) published a theory of fluids that has influenced the designs of airplane wings and sailboats ever since. Exploit this concept to suspend a balloon or ping-pong ball precariously in mid-air with a hair dryer.
What to do: You need a hair dryer and a small round balloon (or a ping-pong ball, depending on the hair dryer’s strength). Turn the hair dryer on, point it upward, and place the balloon in the vertical column of air. If the ceiling is not too high, you should be able to balance the balloon in mid-air this way. Now begin to tilt the hair dryer and watch the balloon stay suspended almost magically.
What’s going on? Everyday experience helps us understand why the balloon or ball stays suspended when the hair dryer is pointed vertically: air blowing upward pushes on the balloon, and this in turn counteracts gravity. But why doesn’t the balloon fall off to the side when we begin to tilt the hair dryer? The answer lies in Bernoulli’s principle, which states that, all other things being equal, a fluid loses pressure as it picks up speed. The air coming out of the hair dryer is moving faster than the room’s air so its pressure is lower. This pressure difference helps keep the balloon suspended, even when you tilt the hair dryer.
Water and cornstarch make a non-Newtonian fluid when mixed: messy but great fun!