Abstract

Objective

In this science project, you will:

1. Develop an understanding of water surface tension,
2. Understand changes in water surface tension under various conditions,
3. Make measurements to estimate the net water surface tension.

Introduction

If you've ever blown up a balloon, you know it's pretty easy to blow up without becoming out of breath when it's made of soft, stretchy rubber. That's because the balloon offers little resistance to becoming stretched out as it is blown up. But for a balloon made of thicker or stiffer rubber, more energy is required to blow it up. The balloon offers more resistance to stretching. Think of a balloon as a model for surface tension. How "stretchy" the balloon material is determines how much resistance (surface tension) must be overcome by the energy of your blowing in order to inflate the balloon. Surface tension is defined as the energy required to increase the surface area by a unit amount.

Liquids also experience surface tension. The molecules (small individual particles) of the liquid experience intermolecular attractions, which simply means the molecules are pulling and pushing away from each other, just like magnets both attract and repel each other. In the case of a water molecule surrounded on all sides by other water molecules, every pulling force is balanced by a pushing force. The net (which means overall) effect is no change. But at the surface, where air and water meet, that isn't true. The water molecules at the surface experience more pulls downward toward the other water molecules below them than upward toward the air. This is the surface tension of the water. You can see it at work when you fill a glass. Even if the water is at the rim of the glass, you can add just a few more drops so that the water is slightly taller than the rim. Don't believe it? Look at the picture below and then try it yourself!

Figure 1. Because of surface tension, the top of this full glass of water curves outward. If more water is added, it will eventually spill over the side of the glass. Note the edge of the glass, and how far the water curves over it.

In this science project, you'll make a small raft from a clear plastic sheet (a transparency), and see if you can propel it by taking advantage of the surface tension of water.

Terms, Concepts, and Questions to Start Background Research

• Resistance
• Surface tension
• Molecule
• Velocity
• Kinetic energy
• Mass
• Acceleration

Bibliography

These websites are good introductions to surface tension:

• Massachusetts Institute of Technology. (n.d.). Introduction to Surface Tension. Retrieved August 12, 2010, from http://web.mit.edu/nnf/education/wettability/intro.html
• Exploratorium. (n.d.). Sticky Water. Retrieved August 12, 2010, from http://www.exploratorium.edu/ronh/bubbles/sticky_water.html

This website offers an explanation of velocity:

• Physics4Kids. (n.d.). Velocity, Speed, and Motion...Oh My! Retrieved August 12, 2010, from http://www.physics4kids.com/files/motion_velocity.html

This website explains kinetic energy:

• Zobel, E.A. (n.d.). Kinetic Energy. Retrieved August 12, 2010, from http://zonalandeducation.com/mstm/physics/mechanics/energy/kineticEnergy/kineticEnergy.html

Materials and Equipment

• Transparencies, like those used on an overhead projector (1 box); available at your local office supply store
• Marker
• Ruler, metric
• Scissors
• Kitchen sponges (1 package)
• Tape
• Lab notebook
• Liquid laundry detergent
• Eye dropper; available at your local pharmacy
• Stopwatch, accurate to 0.01 second (sec)
• Scale, one that can detect small masses in grams (g), such as the Fast Weigh MS-500-BLK Digital Pocket Scale, 500 by 0.1 G, available from Amazon.com
• Liquid hand soap
• Toothpaste
• Toothpicks (1 box)
• Water
• Large basin or sink to hold the water and your transparency raft in early trials
  • It should be large enough that the raft can travel a short distance within it.
  • Make sure it can be easily emptied, as you will be filling and re-filling it with fresh water often.
• Bath tub for later trials
• Volunteer
• 8-inch (in.) x 10-in. picture frame, plastic or wood (the frame will get wet)
• Disposable plastic cups, small (4)
• Zip ties or twist ties

Experimental Procedure

Exploring the Existence of Surface Tension

1. Think of a shape for your raft and draw the shape onto the transparency.
   1. The raft should be symmetric.
   2. The raft should be small enough that it can travel a short distance in your basin or sink, but still be large enough to hold the sponge piece.
   3. You will need to cut a space in the back of the raft where the sponge will be. It should be slightly larger than the size you'll be cutting your sponge (which will be in step 3).
   4. The length of the raft is quite flexible, but we suggest an initial size of 3–5 inches long, and a width that is approximately half of the length you choose. Record the size in your lab notebook.
   5. See Figure 2 for an example of the raft design.




Figure 2. Example of a raft design. Note the way in which the small piece of sponge is attached to the transparency to prevent it from moving and sinking.




2. Cut out the raft.
3. Cut the sponge into small, identically sized pieces. The sponge should fit into the space cut in the back of the raft.
4. Run a toothpick horizontally through one of the small sponge pieces so enough of the two ends of the toothpick can rest on the transparency, then attach the toothpick ends with tape so that the sponge is attached to the small space at the back of the raft. See Figure 2, above. Record the size of the raft and the sponge in your lab notebook.
5. Fill a basin or sink with tap water.
6. Put the raft onto the water surface and let it float.
7. Using an eye dropper, put a drop of detergent onto the sponge at the end of the raft. If one drop isn't enough, put one or two more. The raft doesn't require much detergent to start moving.
8. Observe the motion. Record all observations in your lab notebook.
9. Repeat step 7 with the same raft and sponge, continuing to add detergent a dropperful at a time, in order to become familiar with the effects of the change in surface tension that detergent creates.
   1. Note: After a few trials, the water will have too much detergent in it for its surface tension to change if more is added. When this happens, you will need to replace your water with new water. This will be fairly often.
10. From your background research, you should know that detergents decrease the surface tension of water. How can this help to explain your results?

Exploring Other Substances as Surface-Tension “Motors” for Your Raft

1. Fill up the basin or sink with fresh water.
2. Replace the sponge on the raft with a fresh sponge.
3. Put the raft onto the water surface and let it float.
4. Put a drop of soap onto the sponge at the end of the raft. You could carefully pump one drop, or unscrew the pump lid and let one drop fall from the tube that is attached to the pump onto the sponge.
5. Observe the motion and compare it to the first raft, which used detergent. Record your observations in a data table, like the one below.

6. Repeat steps 1–5. Make sure that you use the same raft for every test, and that you replace the water with new water for every different substance you test. You can try all sorts of substances: vegetable oil, corn starch, salt, and even vinegar! Try out anything that you think might move the raft.
   1. Be sure you have three trials for each substance. This will ensure your results are accurate and repeatable. Record all results in your data table.
7. Which substances worked as a "motor"? Which ones didn't? Which substance was the best motor? Can you relate your findings back to surface tension?

Exploring the Effect of Shape on the Motion of Your Raft

1. Now test different raft shapes. Design another shape for your raft and draw the shape onto the transparency.
   1. You might want to try an asymmetric shape.
   2. You might want to experiment with different sizes, or different basic shapes.
2. Cut out the raft, leaving a space for a small piece of sponge, and insert the sponge at the end of the raft. Again, record all measurements in your lab notebook.
3. Prepare fresh water in the basin or sink.
4. Put the raft onto the water surface and let it float.
5. Put one drop of the substance that worked best in the previous section onto the sponge at the end of the raft.
6. Observe the motion and compare it to the other rafts. Use the same substance for your surface-tension motor for each trial. Record your observations in a data table, such as the one below.
   1. Be sure you have three trials for each substance. This will ensure your results are accurate and repeatable. Record all results in your data table.

Measuring Net Surface Tension

1. Prepare a new raft with a shape similar to the one in Figure 3 below. You will be testing the raft within an 8-in. x 10-in. picture frame, so the raft size is suggested to be 3–5 inches long, and a width approximately half of this length.




Figure 3. Try making a boat with this arrowhead shape. Does it perform better than other shapes you have tested?




2. Weigh the raft using a scale, in grams. This is the boat's mass.
3. You might need more room for your raft to travel this time, so fill a bath tub with tap water.
4. Now build a channel—a barrier sitting at the surface of the water—that will only allow the boat to travel in one direction. The point of the channel is to have a controlled path for the boat to travel along, so you can accurately measure how far it goes. It will travel along the 10-in. length of an 8-in. x 10-in. picture frame. Decide which end will be the launching point, and make marks on the frame that are 25 cm away from the launch point.
5. A perfect object to use for this is an 8-in. x 10-in. picture frame made of plastic or wood. You only want the frame, so remove the glass, backing, etc. Next, tightly connect four small, disposable plastic cups at the corners of the frame using zip ties (or twist ties, as in Figure 4, below). These will be your anchors for your channel, because you want it to remain stationary while the raft travels within the frame. If the frame simply floated at the surface of the water, then it could move around, especially since you will be changing the surface tension.




Figure 4. Use a picture frame anchored by filled cups to keep the channel stationary in your bath tub.




6. Place the channel in your bath tub and fill it so that the frame is at the surface of the water. To keep the channel anchored, fill each cup with water.
7. Set the raft in motion using the substance that propelled your rafts the best from the Exploring Other Substances as Surface-Tension "Motors" for Your Raft section and use a ruler and a stopwatch to measure the time it takes for the raft to travel about 25 cm. You might need extra help for the following steps, so ask a volunteer to hold the ruler while you watch the boat and run the stopwatch.
8. Put the raft at one end of the channel and put one drop of detergent onto the sponge at the end of the raft.
9. Measure the time required for the raft to travel 25 cm inside the channel.
10. Calculate the velocity and the kinetic energy of the boat. Take a look at the Bibliography section to study up on velocity and kinetic energy. The formula for velocity is:

Equation 1:

V =	d  t
V = Velocity d = Distance, in centimeters (cm) t = Time, in seconds (s)




11. Now you need to calculate kinetic energy. You will need your mass to be in kilograms (kg), so convert your grams measurement to kilograms by dividing by 1,000 (for example: if your mass is 2 g, divide by 1,000 to get 0.002 kg). You will also need your velocity to be in meters per second (m/s), so convert your centimeters/second to meters/second by dividing by 100 (for example: if you have 5 cm/s, divide by 100 to get 0.05 m/s).

Equation 2:

KE = (1/2) x m x V2

KE = Kinetic energy, in joules (J) m = Mass, in kilograms (kg) V = Velocity, in meters/second (m/s)




12. Repeat steps 7–11 two more times so you have three trials. You will want to perform three trials per amount of mass you test. Record all data in your data table:

13. Increase the mass of the raft by adding more pieces of transparency. Record the new weight in your lab notebook.
14. Repeat the measurements and calculations for several different weights of the raft.
15. Compare the measured values.

Variations

• How would changing the temperature affect surface tension?
• Try making boats with other materials to see if surface tension is powerful enough to propel bigger objects.
• Try using oil instead of water for the experiment to investigate the surface tension of oil.
• Is the sponge necessary? Try the experiment again, except this time, simply place a transparency raft in water without a sponge in the back, and drop some detergent behind the raft. Does the raft move forward? Is this a more efficient means of propelling the raft, or a less efficient means?
• For another science project on surface tension that includes a more direct measurement of the forces involved, see: Measuring the Surface Tension of Water.
• For a science project on the chemistry of surface tension, see: Measuring Surface Tension of Water with a Penny.

• For more science project ideas in this area of science, see Physics Project Ideas.

Credits

Author: David Chung
Editors: Andrew Olson and Justin Spahn, Science Buddies

Last edit date: 2012-02-07 14:27:32