Surface Tension and 3D Soap Films

Developers:

Pegeen Harper
Sacred Heart School
Mount Holly, N.J.

Kent Carpenter
Modifiers Research
Rohm and Haas Company
Bristol, PA

Grade
Level:

7 and 8

Discipline:

Physical Science (surface tension, bubbles)
General Science (scientific process, testing)

Goal:

Students will be introduced to the concept of surface tension, its dependence on material composition, and how it can produce uniquely shaped surfaces.

Objectives:

1. Students will be able to define surface tension.
2. Students will observe the characteristics of surface tension of water.
3. Students will learn about changes in surface tension with material composition.
4. Students will measure angles on a variety of 3-D soap films.
5. Students will discuss the uniqueness of solutions to a problem.
6. Students will use tests and measurements as a means of comparisons.
7. Students will develop critical thinking and problem solving skills.

Background:

Surface tension at a liquid/gas interface is due to the tendency of a liquid, e.g. water, to want to remain away from the gas, e.g. air. The attraction of the liquid molecules to themselves forces spherical or bubble shaped surfaces which minimize the amount of surface area for a given volume and minimize the number of liquid molecules which must contact the air. The surface tension forces compete with the other phenomena, such as gravitation, which tend to flatten out the spherical surfaces on larger length scales. The surface tension is sometimes thought of as a "skin" on the outer surface of a bulk liquid.

The formation of a gas bubble in a liquid (e.g. bubbles in soda pop or champagne) creates an excess pressure inside the bubble. The pressure enclosed within the bubble is balanced by the surface tension according to the following equation,

P(bubble) - P(bulk liquid) = 2 * surface tension / radius of bubble

An ordinary soap bubble can be thought of as a liquid bubble within a gas with a gas bubble inside the liquid. This makes the pressure inside a soap bubble twice that for a gas bubble within a liquid. The surface tension varies with the composition of the material, e.g. soap at an air/water surface reduces the surface tension between the phases.

When the bubble walls touch, there are always three walls meeting each other at equal angles to form an edge. If the angles are not exactly equal or if a plane is broken, the form is unstable and it cannot last and the bubble shape will re configure itself into a more stable shape. The solution films will slide over one another and never rest until they have settled into a position in which conditions of stability are fulfilled. Amazingly different configurations can result and in some cases the configurations are not unique!

The first two activities (#1 and #2) are designed as an introduction to surface tension. They can be combined with other simple surface tension experiments developed by Donna Hartman and Chuck Jones (Project LABS, 1990) or Carol McGonigle and Beverly El Amma (Project LABS, 1991) to fill out one class period. For best results and understanding of each activity the experiment should be completed by students in pairs, not as demonstrations. The third activity is more involved and is designed for one class period.

References:

Atkins, P.W. Physical Chemistry, Freeman and Co., 1982.

Boys, C.V. Soap Bubbles. Dover, 1959.

Stockard, J., Jr. Experiments for the Young Scientist, Little, Brown and Co., 1964.

Materials:

Activity 1: Surface Tension Experiment

beakers, jars and/or glasses

water

liquid soap

thread

food coloring (optional)

Procedure:

a. Fill a glass with water.

b. Make a loop with thread. Carefully place the loop on the water.

c. Slowly drop liquid soap in the center of the loop.

d. Record your observations.

Questions/
Assessment:

1. From your observations, what is surface tension?
2. Why does the loop of thread float?
3. What happened when a drop of liquid soap was added to the center of the loop of thread? Why did this happen?
4. What other objects might float on the surface of water?
5. Check student's science journal for their understanding of the Scientific Method.

Extensions:

1. Have students discuss instances in their day to day activities where they might have notice surface tension phenomena. (e.g. beads of water on a newly waxed car, droplets of liquid on a spider web or other string, oil/water in a salad dressing.)
2. Have students devise experiments to show surface tension in other liquids, using the Scientific Method. Then have them demonstrate these experiments to their classmates.

Materials:

Activity 2: Surface Tension and Liquid Mixtures

clear glass saucers or small bowls (beakers will also work)

water (liquid A)

rubbing alcohol (liquid B)

food coloring

Procedure:

a. Give each team a container of liquid A and liquid B into which the same color food coloring has already been added.

Note: Remind students never to taste anything in an activity unless instructed to do so by the teacher.

b. Have students put 25ml of each liquid into separate glass saucers. Students should swirl each liquid for a few seconds. Set down the liquids and observe.

c. Pour equal amounts of liquid A and liquid B into a third saucer. Do not use all of each liquid, set some aside for observation comparisons. Slowly swirl this liquid C in the saucer.

d. Set liquid C next to liquids A and B. Observe and compare each liquid. e. Wait a few minutes and observe again.

Questions/
Assessment:

1. What did you observe in liquids A and B when you swirled the saucers?
2. From your observation, were the liquids the same? How can you tell?
3. When you combined the two liquids, what were your observations?
4. What was different about liquid C, then liquids A and B?
5. Knowing that liquids A and B are pure fluids and that the surface tension of a mixture varies with percent composition, what may be causing the phenomena in liquid C. (See C.V. Boys book for further discussion.)

Materials:

Activity 3: 3-D Soap Films

Joy or Dawn dish detergent

Glycerin

Water

pipe cleaners or cloth wrapped floral wire

(Most other types of wire will work. Wire should be stiff enough to hold its shape, but not too stiff that it can not be twisted easily by hand.)

medium size containers (large enough to cover pipe cleaner/wire frames)

Procedure:

a. Make bubble solution shown below. (The teacher can prepare this ahead of time.)

Bubble solution*
1 part dish washing liquid
1 part glycerin
1 part water

Note: This makes a strong solution, with long lasting bubbles. If a large amount of solution is needed use: 1 part detergent, 10 parts water, and 2- 3 ounces of glycerin.

*Solution recipe provided by Dr. Chuck Jones, Rohm and Haas Co.

b. Give each team or pair several pipe cleaners or pieces of floral wire.

c. Have students construct three wire frames shown in the illustrations below. Try to keep the edges smooth and twist the wires tightly together at the corners. The excess wire at the corners can be twisted together and should be pointed away from the inside of the frame. (See illustrations.)

d. Have the students predict what will happen when the frame is dipped into the soap solution and removed. Then completely submerge the frame into the bubble solution, making sure all corners and connecting points are covered.

e. Slowly and carefully lift out the bubble frame. Observe the shapes of the soap films. The most interesting shapes will form when all of the wire edges are in contact with a soap film. (Details of the shapes are described in C.V. Boys book.)

f. Repeat this for each frame. Record observations for each bubble frame.

g. Stretch the frame back and forth without breaking the soap bubble. Note any gross changes in the shape of the soap film configuration.

h. Using a paper clip, break one plane of your bubble. Observe changes and record observations. Break another plane. Observe changes and record observations

Questions/
Assessment:

1. What differences did you observe in the actual bubbles from your hypothesis?
2. How many soap surfaces met on a single wire frame edge?
3. How many soap surfaces met together within the frame to form an edge or line of soap?
4. What degree did each angle measure along an edge where two soap surfaces met? What about where two edges met at a point?
5. What happened when you broke the first plane of your bubble? The second? Why?
6. What would happen if you continued to break the planes ? Why?
7. Are the soap surfaces formed in the pyramid frame and the cube frame planar or do they have curvature like an ordinary soap bubble?
8. Are the soap surfaces formed in the spiral frame planar or are they curved? (After answering questions 7 and 8, have the students review the equation given in the background section. Is the pressure on both sides of the soap film the same or different in these soap films (versus an ordinary soap bubble)?)
9. Does the bubble pattern look the same every time the frame is extracted from the solution? (Even when all the frame is touching the soap solution?) What if a bubble on top of the soap solution is dragged out and trapped inside the soap film?
10. (Referring to question 9) Knowing that surface tension is trying to minimize the area required to form the soap film in order to make the most stable, lowest energy configuration, do you think different configurations all have the same minimum energy? What does this say about the uniqueness of the configurations formed and why do configurations which may have higher energy form in one case and not in others?

Extension:

1. Have students create their own frame designs. Are the results the same? In what way? How are they different? What problems did the students face when creating their own? How can those problems be modified to make their frames work?
2. Language Arts/Art Divide class into small groups. Each group is to write a story about a day in the life of a bubble. It should be designed for the younger grades. The story should follow a bubble from the time it was made, through its adventures of the day, until it eventually pops. The story should also be illustrated. Some students in the group will be the writers, other students will work on the illustrations and book cover. When finished, the group could read the books to their reading buddies or younger classes.