SWIRLING MILK
INTRODUCTION
PURPOSE
The purpose of this activity is to illustrate awesome, colorful, behavior using three very simple ingredients. This activity can also be used as a research project for older students.
MATERIALS
PROCEDURE
OBSERVATIONS
Initially, the food colors swirl in different patterns across and under the surface of the milk for several minutes. Note that the colors do not actually mix with each other, but continue in separate swirling patterns. The time of swirling may depend on the temperature of the milk and amount of dishwashing liquid you use. You can repeat step 7 to reactivate the swirling motion if needed. Moving the toothpick to a new spot can later result in mixing of the various colors.
EXPLANATION
The reason for the swirling motion of the food colors must have to do, among other factors, with the changes in surface tension of the milk after adding the drop of liquid dishwashing detergent. A characteristic of detergent molecules is that they have a partially charged polar end and an uncharged nonpolar end. Thus, they can bind weakly to either polar or nonpolar neighboring molecules. When detergent is introduced into the center of the bowl, it may bind weakly to the water in the milk, decreasing its surface tension so that it flows more easily, and indirectly setting up the swirling motion. This could explain the behavior observed by us and others that skim milk allows or causes more swirling than whole milk. Skim milk has less fat, is less viscous or less resistant to flow, and can react more easily to changes in surface tension of the liquid. However, this cannot be the whole story, as indicated by the often contradictory explanations that have been suggested to us by surfactant chemists who should thoroughly understand the functioning of the detergent.
Changes in temperature certainly cause a difference in the degree and speed of swirling. The swirling is slow with cold milk, but speeds up as the milk is warmed. Changes in density of the liquid may also have an effect. The issue is further complicated by the fact that several of these factors are interrelated. Try this activity for yourself, and send us what you think are the reasons for the observed behavior, especially ones that can be explained in layman’s terms.
ADDITIONAL SUGGESTIONS
This activity can be used at any grade level. Pre-K children are fascinated by the behavior, and will perform the experiment again and again, always with different patterns. They usually show their parents because we hear from parents later about stopping at the grocery store on the way home to pick up "Dove" or food coloring. However, there are so many variables in this activity that it could just as well comprise a research problem for college students. You can suggest to students that they try this activity at home, using whole, 2%, 1%, skim, and reconstituted dry milk (or even orange juice, vinegar, oil, ginger ale, etc.) as well as trying a whole series of liquid dishwashing detergents. Have them keep careful notes and even bring photographs to school to show their observed behavior other students. We have found, for example, that “Joy” does not work nearly as well as others even though "Joy" is the detergent of choice for many bubble experiments. Children can also study behavior at various temperatures of milk.
The various patterns can be saved in part. When a desired pattern is present (and before thorough mixing gives a dull, gray-green, color), carefully lower a piece of filter paper, paper towel, or tissue onto the milk surface. Then remove the filter paper, towel, or tissue, and lay it on a flat surface such as a cookie rack, or hang it over a sink to dry. The colors will flow somewhat if it is hung vertically, and it tends to look like a hot-air balloon, but striking patterns are obtained in either case. Some teachers laminate these patterns to preserve them.
Reference
|
||||||
