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Project Summary

Difficulty	4
Time required	Short (several days)
Prerequisites	None
Material Availability	Readily available
Cost	Very Low (under $20)
Safety	No issues.

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Abstract

Objective

This project has three objectives:

1. develop a qualitative 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

You probably have experience blowing up a balloon. For a balloon made of soft, stretchy rubber, it is easy to blow it up without becoming out of breath. 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. We can 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 of the liquid experience intermolecular attractions, the details of which depend on the chemistry of the particular liquid. The molecules at the surface of the liquid experience asymmetric forces, since one side of the molecule faces the liquid, and the other side faces air. This asymmetric force is surface tension.

Water may seem ordinary, but is has some extraordinary chemical attributes. One of these attributes is hydrogen bonding. To really understand the surface tension of water, you should do some background research on the chemical structure of water and hydrogen bonding.

The effects of surface tension can be observed easily in our daily life. For example, soap bubbles, water rising inside a capillary tube, floating a needle on a still water surface, floating of water strider on water, using detergent to clean up kitchen utensils, and many others. At the smaller scales of microscopic or nanoscopic worlds, the forces of surface tension play an increasingly important role determining the behavior of molecules.

In this project, you'll make a small boat from a clear plastic sheet (transparency), and see if you can propel it by taking advantage of the surface tension of water. All right, it's more of a motor-raft, but maybe you can think of a good way to make a lightweight boat for this project.

Terms, Concepts and Questions to Start Background Research

To do experiments on surface tension, you should do research that enables you to understand the following terms and concepts:

• surface tension
• water molecules
• hydrogen bonding of water molecules
• detergent
• capillary motion
• acceleration
• velocity

Bibliography

• Origin of surface tension:
  http://wine1.sb.fsu.edu/chm1045/notes/Forces/Liquids/Forces03.htm
• Information on bubbles and surface tension:
  
  • http://www.exploratorium.edu/ronh/bubbles/bubbles.html and
  • Noddy, Tom. Tom Noddy's Bubble Magic. Philadelphia, Pennsylvania: Running Press, 1988.
• Information on acceleration, velocity, and force:
  http://physics.webplasma.com/physicstoc.html

Materials and Equipment

• transparency
• scissors
• several cotton balls, cotton swabs or small pieces of sponge
• marker
• ruler
• stop watch, accurate to 0.01 s
• postal weighing scale
• glue or tape
• liquid detergent
• soap
• toothpaste
• water
• thermometer (optional)
• a few small pieces of wood to make a straight channel for your raft

Experimental Procedure

1. Exploring the existence of surface tension
   1. Think of a shape for your raft and draw the shape onto the transparency.
      • The raft should be symmetric.
      • You will need a slot should at the end of the raft in order to insert a piece of cotton or sponge for the storage of "fuel."
   2. Cut out the raft and insert the cotton or sponge in the slot.
   3. Prepare fresh water in a bathtub or basin.
   4. Put the raft onto the water surface and let it float.
   5. Put a drop of detergent onto the cotton or sponge at the end of the ship.
   6. Observe the motion.
      
   From your background research, you should know that detergents decrease the surface tension of water. How can this help to explain your results?
   
   
2. What other substances can work as a surface-tension motor for your raft?
   1. Prepare fresh water in a bathtub or basin.
   2. Change the cotton or sponge of the raft.
   3. Put the raft onto the water surface and let it float.
   4. Put a drop of soap or toothpaste or some other substance onto the cotton or sponge at the end of the ship.
   5. Observe the motion and compare to the first raft.
   
   
3. Exploring the effect of shape on the motion of your raft
   1. Design another shape of a ship projection and draw the shape onto the transparency.
      • You may want to try an asymmetric shape.
      • You may want to experiment with different sizes, or different basic shapes.
   2. Cut out the raft and insert the cotton or sponge at the end of the raft.
   3. Prepare fresh water in a bathtub or basin.
   4. Put the raft onto the water surface and let it float.
   5. Put a drop of detergent onto the cotton or sponge at the end of the ship.
   6. Observe the motion and compare to the other rafts.
   
   
4. Measurement of Net Surface Tension
   1. Prepare a new raft with a shape similar to the one below:
      
      The length of the raft is quite flexible but for the first trial, it is suggested to be 3–5 cm long, and a width approximately half of this length.
   2. Weigh the raft.
   3. Prepare the a channel for your raft, marked with a predetermined distance (see drawing, below):
      The length of the channel is suggested to be about 25 cm long with the width of the channel just slightly larger than the width of the raft. The height is about 3 cm. If you make bigger boats, you'll need to adjust the dimensions accordingly.
   4. Prepare fresh water in a bathtub or basin.
   5. Put the raft at one end of the channel and put a drop of detergent onto the cotton or sponge at the end of the ship.
   6. Measure the time required for the raft to travel inside the channel between the predetermined marks.
   7. Calculate the acceleration and the kinetic energy imparted to the boat.
   8. Increase the mass of the raft by adding more pieces of transparency (record the new weight).
   9. Repeat the measurement and calculations for several weights of the ship.
   10. Compare the measured values.

Variations

1. How would changing the temperature affect surface tension?
2. You could try making boats with other materials to see if surface tension is powerful enough to propel bigger objects.
3. You could try using oil instead of water for the experiment to investigate the surface tension of oil.
4. For another project on surface tension that includes a more direct measurement of the forces involved, see: Measuring the Surface Tension of Water.
5. For a 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
Editor: Andrew Olson, Science Buddies

Last edit date: 2005-12-14 14:27:32

Career Focus

If you like this project, you might enjoy exploring careers in Physics.

	Physicist Physicists have a big goal in mind—to understand the nature of the entire universe and everything in it! To reach that goal, they observe and measure natural events seen on Earth and in the universe, and then develop theories, using mathematics, to explain why those phenomena occur. Physicists take on the challenge of explaining events that happen on the grandest scale imaginable to those that happen at the level of the smallest atomic particles. Their theories are then applied to human-scale projects to bring people new technologies, like computers, lasers, and fusion energy.			Nuclear Monitoring Technician Nuclear technology is used to image the human body, destroy cancer cells, sterilize food and medical equipment, create pest or drought-resistant seeds, and to generate power for 1 in 5 U.S. homes and businesses. Nuclear monitoring technicians help to keep the people who work with nuclear technology and the environment safe from excessive radiation exposure. They use special instruments to measure and monitor the radiation levels of workers, work areas and equipment, and they are involved in decontaminating work areas to safe levels. They also educate workers on radiation safety.
	Nuclear Medicine Technologist Many traditional medical imaging methods, like X-rays, can take pictures of certain parts inside the body, but sometimes these methods are not sensitive enough to detect a problem, or a picture is not enough—the doctor needs to see how a part is functioning, not just how it looks. That’s where nuclear medicine comes in. It can be used to see, for example, if bone repair is going on in a certain area, how a kidney is functioning, how a stomach is emptying, or how blood is flowing into and out of a heart. It can also be used to treat certain diseases. Nuclear medicine technologists are the special healthcare workers who administer radioactive drugs, take images of the patient, and then process, analyze, and show the computer images to the doctor.			Forensic Science Technician Guilty or not guilty? The fate of the accused in court lies with the evidence gathered at the crime scene. The job of the forensic science technician is to gather evidence and use scientific principles and techniques to make sense of it. It can be a grueling and graphic job, but very rewarding. If you like the idea of using science to help deliver justice, then you should investigate this career.

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