Soap Films in Action
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Description

Several different frames are dipped in soapy water to illustrate the tension present in soap films. The films make a membrane-like structure which tends to minimize its area.

Chemical Concepts

Intermolecular forces in liquids can do work. These forces are an important part of many biological membranes.

Safety

Clean up any spills with paper towels. Soapy solution can be very slippery.

Procedure

  1. To illustrate the intermolecular forces, dip a metal ring (a hanger works well) in soap solution. Wave the ring back and forth gently along a line perpendicular to the plane of the ring.

  2. Tie a circle with thin string or heavy thread leaving one end about 20 cm long. Tie the string from the hook of the metal hanger.

    Ring

  3. With one finger hold the circle of string open while dipping the entire metal ring in soap solution. Poke a hole in the middle of the string circle with a different finger. Intermolecular forces in the liquid pull the string into a perfect circle. Swing the metal ring gently in the plane of the ring. Note the motion and shape.

  4. Coil a hanger in a screw pattern around a soup can, or beaker leaving 30-40 cm on the end straight. Remove the can, and tuck the straight end down through the center of the coil. Slip two small sections of tubing (a drinking straw may be substituted for the tubing) onto the center rod, then twist the two ends together to make a handle. Slip one of the sections onto the spiral, and keep the other on the center rod. Tie a 30 cm length of thin string or heavy thread onto the section on the center rod, leave a little slack, and then tie the string around the straw section on the spiral. Leave the rest of the string dangling.

    SpirC

  5. Wind the string to the hook or handle end. Dip the entire apparatus in soap solution. Remove and slowly move the string up the coil closing the film of soap. Reverse direction. Work slowly and steadily.

  6. Cut out the middle of a 2-L bottle, leaving 5 cm of the side walls on the bottom. Cut just enough off the top section so that it fits easily into the bottom. Pour soap solution into the bottom of the bottle. Dip the top into the solution. Slowly lift up the top 3-4 cm, and observe the soap film that forms between the two circular rims. Observe what happens to the film as you lift the top up further. Put the top back inside the bottom, and begin lifting it slowly again, but this time, blow air gently into the mouth of the bottle top as you lift. Note: you need not put your lips on the bottle, Just blow gently from 15-20 cm away. Observe what happens to the film as you lift the top up further. Sway the top to make the tubular film dance a little.

  7. Dip a slinky. Support both ends of the slinky. Dip into the soap solution. Lift both ends of the slinky and slowly pull.

Questions

  1. What dictates the precise shape of soap films that form when a given frame is dipped into soapy water?
  2. What forces are acting on the soap film that might affect its shape? What shape does the soap film on the metal hoop assume? Why? Why is the shape not perfect?
  3. What shape does the noose assume when the soap film inside it is popped? Why does it assume this shape?
  4. If the shortest distance between two points is a straight line, then you might think that the minimum surface to connect the upper and lower bottle rims should be a straight cylindrical film between them. Why is this not the case?

Handout

Name ___________________________ Class ________

Teacher __________________________

BeckerDemos 028 Soap Films in Action

Watch the movies and answer the questions.

Curriculum-

Use this activity when discussing intermolecular forces.

Activity-

Demonstration - Student or Teacher

This activity works best as a brief demonstration.

Safety-

Soapy water may be very slippery. If students are dipping the models, watch for spills. Keep paper towels on hand for them to clean up.

Time-

Teacher Preparation: Make soap solution a day ahead. Model construction varies. Most require only a few minutes and may be reused many times.

Class Time: 5 minutes

Materials-

Disposal-

The solids may be discarded with ordinary solid trash.

Lab Hints-

Answers-

Q1. What dictates the precise shape of soap films that form when a given frame is dipped into soapy water?
A1. The soap films achieve their lowest potential energy when the surface area they cover is minimized, for this always allows the greatest degree of hydrogen bonding to occur between adjacent water molecules. Each configuration of films shows the minimum surface area required to cover and interconnect the complete frame.
Q2. What forces are acting on the soap film that might affect its shape? What shape does the soap film on the metal hoop assume? Why? Why is the shape not perfect?
A2. Surface tension, caused by intermolecular attractions is the major force. Gravity is also acting on the soap film. The film is essentially flat because a flat film covers the frame using a minimum amount of surface area. This allows for maximum intermolecular attractions. Essentially, any distortion from a flat plane separates water molecules from one another, and is therefore less stable. The film is not perfectly flat, however, especially when it is held in a horizontal plane, since gravity pulls it down into a very slight bowl shape.
Q3. What shape does the noose assume when the soap film inside it is popped? Why does it assume this shape?
A3. The noose becomes almost perfectly circular. Again, because the surrounding soap film is most stable when it is least stretched, the circular hole allows for the surrounding film to have the smallest surface area possible.
Q4. Draw the shape of the soap film that forms when the two bottle halves are pulled apart. If the shortest distance between two points is a straight line, then you might think that the minimum surface to connect the upper and lower bottle rims should be a straight cylindrical film between them. Why is this not the case?
A4. Consider the film connecting the upper and lower rims to be made of a bunch of thin vertical strips. For any of these strips, a straight vertical line would obviously provide the least stretch. (Consider that a rubber band always becomes straight when it is stretched). Yet by curving inward the way the film does, each strip can become considerably thinner around the center, thus minimizing the horizontal stretch. This decrease in surface area around the middle must be greater than the longitudinal increase caused by the curvature of each strip, and so the film bends inward into a shape known as a "catenoid." A catenary is the shape assumed by a slack rope that is hung between two points. When this shape is turned 90 degrees and then rotated about a vertical axis with its curve opening outward, this forms a catenoid. Interestingly, although it doesn't look like it, a catenoid is actually comprised entirely of straight lines just as a cylinder is; but its straight lines extend diagonally between the two rims. A model of this can easily be constructed by joining the two rims with a series of rubber bands. If the top is lifted, the rubber bands form a cylinder made of straight vertical lines. If the top is lifted and given a slight twist to one side, the rubber bands, each of which is still straight, form a catenoid.

References-

Acknowledgements:

Many of the ideas described above came to me while reading the wonderful book by C. V. Boys: Soap-bubbles, Their Colours and the Forces Which Mold Them. Dover Publications, Inc., 1959.

Further Reading:

Key Words 1-

intermolecular interactions


This static was created at 1:47:46 PM on Thursday, April 16, 2015