WEEK 12: Surface Tension.
This ended our series on flotation/buoyance/density/volume. The children tried to make a needle float, which you can't do by just placing it on top of the water. However, if you place it on a small piece of paper towel, the paper towel and needle will float, but then the towel will soak up water and sink, leaving just the needle. The needle defies all we've been learning about densities, buoyancy and volume, because there's another property: surface tension. Surface tension describes the attraction between the surface water molecules which causes the surface of a liquid to act like a thin skin stretched across it.

We saw that water can actually rise above the rim of a cup if we put small objects one at a time into a full cup.

Applications: don't touch the tent! Surface tension will bridge the holes in the fabric of a tent when it rains, but if you touch it, you will break the surface tension, enabling the rain to drip into the tent.

Clinical test for jaundice uses surface tension of urine. Normal urine has a surface tension of about 66 dynes/cm but if bile is present (a test for jaundice), it drops to about 55. In the Hay test, powdered sulfur is sprinkled on the urine surface. It will float on normal urine, but sink if the S.T. is lowered by the bile.

We also used food coloring on top of water to "see" surface tension being broken by an object touching the water surface.

Then we added the concept of a surfectant. Here's what we did:

1. Pour enough milk in a dinner plate to completely cover the bottom. Allow the milk to settle.

2. Add one drop of each of the four colors of food coloring - red, yellow, blue, and green - to the milk. Keep the drops close together in the center of the plate of milk.

3. Find a clean cotton swab for the next part of the experiment. Predict what will happen when you touch the tip of the cotton swab to the center of the milk. It's important not to stir the mix. Just touch it with the tip of the cotton swab. Go ahead and try it.

4. Now place a drop of liquid dish soap on the other end of the cotton swab. Place the soapy end of the cotton swab back in the middle of the milk and hold it there for 10 to 15 seconds. You'll see an amazing color show.

5. Add another drop of soap to the tip of the cotton swab and try it again. Experiment with placing the cotton swab at different places in the milk. Notice that the colors in the milk continue to move even when the cotton swab is removed. What makes the food coloring in the milk move?

How does it work?

Since milk is mostly water, it has surface tension like water. The drops of food coloring floating on the surface tend to stay put. Liquid soap wrecks the surface tension by breaking the cohesive bonds between water molecules and allowing the colors to zing throughout the milk. We call things that mess up the surface tension of water SURFECTANTS.

There's another reason the colors explode the way they do. Milk is mostly water but it also contains vitamins, minerals, proteins, and tiny droplets of fat suspended in solution. Fats and proteins are sensitive to changes in the surrounding solution (the milk).

When you add soap, the weak chemical bonds that hold the proteins in solution are alteredn The molecules of protein and fat bend, roll, twist, and contort in all directions. The food coloring molecules are bumped and shoved everywhere, providing an easy way to observe all the invisible activity.

At the same time, soap molecules combine to form a micelle, or cluster of soap molecules. These micelles distribute the fat in the milk. This rapidly mixing fat and soap causes swirling and churning where a micelle meets a fat droplet. When the micelles and fat droplets have dispersed throughout the milk the motion stops.

If you want to so more at home:

Hypothesize what would happen if you repeated the experiment using water in place of milk. Will you get the same eruption of color? Why or why not? What kind of milk produces the best swirling of color: skim, 1%, 2%, or whole milk? Why?

Application: why detergent pollution is bad – soap breaks bond between oil and water – that’s why we use soap to clean dishes, but birds depend on this. If water fowl come into contact with soap, they lose their waterproofing, water soaks them, they get heavier and sink.

Why we should wash our hands with soap: our hands have oils that can't be broken down by just water - we need a surfectant - soap - to break down the oils that may be trapping dirt and germs, which leads us to...