Re: Why Did the Milk Swirl Around?

Jenna,

When the food color is placed on the surface of the milk, the drops remain intact with little spreading. This is due to the fact that the water-based food color does not mix with milk easily. This is because the milk is a suspension of fat globules in water, and food coloring is a water-like dye. In general, fat and water do not mix. This is why the food coloring stays in a drop.

When the soap is added to the milk, the soap spreads out over the surface and causes the food color to swirl about. It is a little complicated on how this works, but I will try to do my best for you. Also, I have a copy of the lesson, and if you would like, I could mail it to you. Just send me an e-mail (to raber@lebmofo.com) with your address and I'll send it. Anyway, here goes my explanation.

Soap molecules have two different parts. One longer part that is like fat and oil, and one shorter part that is like the water. The longer part is called the nonpolar or hydrophobic (water hating) part, and the shorter part is called the polar or hydrophillic (water loving) part. The longer fat like part of the soap joins with the fat in the milk, and leaves the shorter part sticking out into the water. When this happens, it forms what is called micelles. Micelles look much like oranges with cloves poked though the rinds. Part of the swirling and churning motion you see when you add soap to your dish of milk is a result of the attraction (or liking) between the soap molecules and the fat molecules of the milk.

As the soap spreads across the surface of the milk, more and more fat globules are pulled to the spreading soap. As these fat globules congregate, the water in the milk is pushed away, taking the food coloring with it. The movement of the milk will decrease as the soap becomes "used-up" (forming the micelles). The addition of more soap can often reactivate the swirling and churning motion. Eventually however, the system will reach what is called equilibrium (uniform distribution of fat globules-soap micelles), and the motion will stop.

If you don't understand this completely, don't feel bad. I know some college seniors that had a hard time with this topic in our Biochemistry class. If you need any further explanation or would like the complete lesson plan, please e-mail me at raber@lebmofo.com.

-Jeff