I told Squirt at the science open house that we'd do some science-y stuff after school today. Wanting to make good on that, I decided to pull the out the 'ol Oobleck trick. This stuff never ceases to amaze kids. It's a common preschool activity but I've used it effectively to illustrate points in my High School geology classes. I'm telling you, regardless of age people can't seem to let this stuff sit untouched.
The "recipe" is simple:
Cornstarch + water + food coloring (if you want) = Oobleck
Add the cornstarch to water carefully, mix to combine.
Get your hands on in there! That's what this stuff is for.
Notice how you can hold it firm and it compresses into a "ball," but when you let go it flows like a liquid. Awe. Some.
Ok, so the amounts matter...
Aim for a 1:2 ratio of water to cornstarch. If this off a little, add a little more cornstarch or water until you get a nice consistency. It should be stiff, almost dry, when you apply pressure and flow easily when you release the pressure. Add food coloring with the water if you like.
You'll end up with a goo that is really fun to play with. Grab a handful and squeeze it into a ball. Feels pretty solid, doesn't it? Now hold your hand open and see what happens to the ball. Spread your fingers out a little. If you made a large bowl of Oobleck, try letting it all drip back into the bowl and punching it. Were you able to make much of a dent in the goo?
FYI: This is messy. When you are done, DO NOT POUR OOBLECK DOWN THE DRAIN. If you put your oobleck in a container and let it sit, you will notice that the cornstarch settles to the bottom into a thick immoveable block. If you pour it down the drain this will happen in your pipes causing untold damage and frustration... just toss it into the trash when you're finished.
As a mom this is how I explain it:
All those little bits of cornstarch are swimming around in the water. When you push hard on it, they sort of lock together and can't move around anymore. It's like they all link arms and hold onto each other. That's why you can squish it into a ball and it will feel hard. If you let go, it's like they all let go of each others arms and all those little bits of cornstarch can now move wherever they want. Watch as it drips and you can see the cornstarch floating in the water.
As a teacher this is how I explain it:
Oobleck is something called a Non-Newtonian fluid which flows differently under different conditions of stress (pressure). Oobleck is a dilatant Non-Newtonian fluid which means it thickens or becomes less viscous under stress. Some other non-Newtonian fluids you may be familiar with are ketchup, paint, and human blood. Ketchup is something called thixotropic, it becomes less viscous when you apply pressure. You have probably noticed that ketchup tends to thicken as it sits. Shaking the bottle or stirring it with a knife applies a stress that makes the ketchup less viscous so it will pour more easily. This is what non-Newtonian fluids do. As a result of applied stress (pressure) they flow differently. Sometimes the difference is hard to recognize without complex measurements, but it's pretty easy to tell when ketchup doesn't come out of the bottle...