Liquid Science

INTRODUCTION:

Cornstarch and water forms a slurry that acts as a Non-Newtonian or "dialatent" fluid. It acts like a viscous fluid in that it will flow slowly when cool and more quickly when warmed. In the correct proportions, however, the starch granules are extensively hydrogen-bonded to each other by interstitial water molecules. This prevents the substance from responding to quick changes. Apply a shear stress and it will appear to break like a solid.

This demonstration uses this resistance to rapid change to make a point about "studying".

CHEMICAL CONCEPTS:

-Viscosity
-Thixotropic and Dialatent fluids
-hydrogen bonding

MATERIALS:

-cornstarch (one 16 oz box)
-2 cups (500 mL) water
-large mixing bowl or other rigid container
-large zip-lock bag
-one 1 L beaker

PROCEDURE:

Add about half the box of cornstarch to the bowl and slowly add 1 cup of water, mixing as you add the water. When the mixture is all the same consistency, add either more starch or more water to create a fluid that will flow slowly through your fingers, but can be broken off and rolled into a ball. The ball should remain stable if kept in motion but should start to flow when left in the palm of your hand.

Store the mixture in a large zip-lock bag. (The mixture WILL grow mold if left for more than a day or two.)
When you are about to do the demonstration, knead the bag to fully mix the suspension and, in front of the class, pour it from the bag into the 1 L beaker.

Explain that this is "Liquid Science". Dip your fingers in it and pull them out. They will be covered in the white mixture. Point out that "Science knowledge" sticks to anyone who dabbles in it. Once they start learning Science, it is easy to "get immersed" in the subject!

It is most effective to have a student act as a demonstrator for the next part.

After you have made the analogy with trying to "cram for a test" all at once, ask the student to hit the top of the mixture hard with two fingers held together. The volunteer will be tentative at first, but will soon try to hit it with more and more force. The harder they hit, the less able they will be to penetrate the surface.

Now ask them to let their fingers drift down into the liquid, all the way to the bottom. Watch the expression on their face as the fingers glide effortlessly into the "white solid"!

When the hand is removed, "lots of knowledge" is stuck to it, without any apparent effort on their part!

At this point the rest of the class will want to try it. Bring out smaller zip-lock bags with about 200 mL of premixed starch suspension. The bags can be placed into a 250 mL beaker, then opened. The liquid will stay inside the bag but will flow to take the shape of the bottom of the beaker. With the top of the bag folded over the outside of the beaker, students will be able to experience the demonstration without creating too much mess.

SAFETY PRECAUTIONS:

[These suggestions are NOT intended to be a complete review of all the safety issues involved with this activity. Professional judgement and practices are essential. If you are unsure of the safety precautions that should be taken, seek experienced assistance.]

[NOTE: Just like Science, this mixture sticks to those who dabble in it. Be prepared for clean-up of hands afterward. ]
Drops on the floor will dry and can be swept up later. Warn your students and any cleaning staff that the dry powder can be very slippery on the floor!
The starch will fall out of clothing when dry.

DISPOSAL:

The starch is not permanently suspended in the water. The mixture must NOT be washed down a drain, since it can form a gel if heated and can plug drain pipes.

Dispose of it in regular waste paper trash containers.

Hands can be washed off with soap and lots of warm water.

DISCUSSION:

This is a simple, but very powerful demonstration. If the class is allowed to play with the mixture, they will have a concrete image of the futility of studying short-term, and the advantages of doing their work in a regular and consistent way. One exposure to the effect, and you have an experience you can refer back to time and again. Your students' own logic will be working for you (and them) when you reinforce good study habits.

Corn starch is used because it is almost 100% starch, with very little protein. The starch is in microscopic granules, which form hydrogen bonds to water. [A mechanical analogy would be a bunch of popcorn kernels mixed together with a small amount of corn syrup.]

Cooks will heat a dilute starch-water mixture to about 65^o - 70^oC, where the granules have absorbed enough water to lose their internal organized structure. They become tiny clear gels. As the granules absorb more water, hydrogen bonding in the liquid increases and a "sauce" begins to thicken. [This is not unlike the effect of "popping" the kernels of popcorn in the above analogy. More surface area would allow the syrup to stick more effectively.]

ACKNOWLEDGMENT:

Harold McGee, On Food and Cooking, C. Scribners & Sons, ISBN0-684-18132-0
(an amazingly useful book!)