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Measuring Surface Tension
««« From http://www.teachengineering.org/
Contributed by: NSF CAREER Award and RET Program, Mechanical Engineering and Material Science, Pratt School of Engineering,
Duke University. Used with permission, as stated in Terms of Use on site.
Curriculum Connection: Physics 12C (Hydraulic and Pneumatic Systems Unit)
Pre-Requisite Knowledge
Students should have the ability to manipulate algebraic equations, and calculate averages and standard deviations.
Learning Objective
After this activity, students should be able to:
• Describe how the combination of adhesive and cohesive forces causes water to rise in a thin tube (capillary action).
• Discuss the role of capillary action in measuring surface tension.
• Practice using scientific notation and calculating experimental error.
Materials List
For the introductory teacher demonstration:
• a can of paint, motor oil or other fluid
•
a container into which you can pour the above fluid
Each group needs:
• 1 ring stand with support ring attached
• capillary tubes of various diameters
• large Petri dish
• twist-ties
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water and dark food coloring
ruler
eye protection for each student
Measuring Surface Tension Worksheet
one per person (see Appendix 1)
Introduction/Motivation
(Open a can of paint or motor oil or other fluid, and pour into a container or otherwise demonstrate its properties.)
Ask: What are some important properties of paint (or whatever liquid you are using)? What do we want to accomplish
with the paint? What does it need to be able to do? Encourage students to think about what would happen if paint
spread like plain water on a wall. The surface tension is too high and you cannot get the paint to spread evenly.
On the other hand, if the surface tension is too low, the paint layer will be too thin and the surface under the paint will
show through. If any students have painting experience — artistic or home improvement — encourage them to describe
any problems they have had that are related to this surface tension discussion.
For liquids used in coatings, their surface tensions are extremely important to get just right so they work correctly.
One of the most accurate ways to measure surface tension is to use capillary action, the ability for a fluid to climb
a thin tube. In this lab activity, we will use capillary tubes in order to measure the surface tension of water.
Measuring Surface Tension
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Student Activity
Measuring Surface Tension
Procedure
These procedures are written with advanced students in mind. They can be modified for use in a general class.
Before the Activity
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Gather materials and make copies of the Measuring Surface Tension Worksheet at the end of this article.
Organize lab stations.
Mix food coloring with water and distribute to each station.
If lab time is limited, attach the capillary tubes to the ring stands above the Petri dishes before class
(see Figure 1).
Figure 1: The height water will rise in a thin tube is determined
by the material of the tube, the diameter of the tube, and the
surface tension of the climbing liquid.
Copyright © 2010 Jean Stave
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Student Activity
With the Students
1. Divide the class into groups and send them to the lab stations.
2. Direct students to follow the worksheet lab instructions, recording data and answering questions.
3. Have students carefully attach the capillary tubes to the support rings on ring stands in increasing order
of their inner diameters.
4. Have groups each place a large Petri dish beneath the capillary tubes and lower the tubes so that their
ends are just above the bottom of the dish (see Figure 1).
5. Have students fill the Petri dishes with dyed water and watch the water rise in each tube. Expect the
water to reach its maximum height quickly (2– 3 minutes).
6. Have groups measure the height of the water in each tube and record their data.
7. Have each student calculate the surface tension found experimentally for each tube.
8. Have each student calculate the average surface tension overall and the standard deviation.
9. The accepted value of the surface tension of water in air at 20 °C is γ = 0.073 J/m2. Have students
compare their measured values for surface tension with the accepted values and speculate on why
the values are different.
10. Assign students to individually research a substances for their surface tensions and applications,
as described in the Assessment section. Follow-up in a later class period to share findings.
Safety Issues
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Have students wear eye protection since glass is used in this experiment.
Handle the capillary tubes with care! The tubes used in this experiment have a thick outer wall, but the
glass is still fragile and can be easily broken if not treated gently.
Troubleshooting Tips
Important: The measurement of surface tension depends greatly on:1) the purity of the water, and 2) the cleanliness
of the capillary tubes. (See pg. 6 in Capillarity and Wetting Phenomena by Pierre-Gilles de Gennes, et al., for a short
discussion.) Students’ results should still agree within an order of magnitude with the accepted value, but will probably
be less than the accepted value.
• To see the dyed water in the thinnest tubes, dye the water very dark. If students still have trouble seeing
the water, suggest they place a piece of white paper directly behind the capillary tubes.
• Capillary tubes are fragile. Depending on students and time constraints, you may want to set up the capillary
tubes and large Petri dish (without water) before class.
• Clean capillary tubes as soon as possible after class by submersing them in water and then blowing air
through them.
• On occasion, water rises significantly different amounts, depending on which side of the capillary tube is
placed in the water. If possible, check tubes before class or number the capillary tubes given to each group
and note if any seem to behave oddly.
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Assessment
Pre-Activity Assessment
Brainstorming: In small groups have students agree upon three applications in which knowing the surface tension of
a product is important in quality control. (Anywhere a liquid needs to adhere to a surface in a particular way will depend
on surface tension. Examples include: Painting cars or aircraft, cleaning with soap and water, and applying sunscreen
or moisturizer.)
Activity Embedded Assessment
Activity Questions: Students demonstrate their thought processes during the activity by answering the questions on the
Measuring Surface Tension Worksheet. They extend their learning to the real-world problem of measuring the surface
tension of a liquid to ensure proper application in an industrial process. Gauge student understanding by circulating
the classroom and asking how they answered specific worksheet questions.
Exploring Further
Assign students to individually research the surface tensions reported online for various substances, and report on
applications for which these substances are used.
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References
Adison, Arthur W., et al. Physical Chemistry of Surfaces. New York, NY: Wiley, 1997, p. 16-19.
Brown, Theodore, et al. Chemistry: The Central Science. 9th edition. Upper Saddle River, NJ: Pearson Education, Inc.,
2003. (General information on surface tension and capillary action.)
de Gennes, Pierre-Gilles, et al. Capillarity and Wetting Phenomena: Drops, Bubbles, Pearls, Waves. New York, NY:
Springer, 2004.
JRank Science & Philosophy Science Encyclopedia. “Capillary Action.” Science.jrank.org. Accessed June 2010.
http://science.jrank.org/pages/1182/Capillary-Action.html/.
Mike. “Tree Physics 1: Capillary Action, the Height of Trees, and the Optimal Placement of Branches.” Posted July 2009.
Npand.wordpress.com. Accessed August 2009. (Derivation of water height in capillary tubes.)
http://npand.wordpress.com/2008/08/05/tree-physics-1/.
Robinson, Clay. “Capillary Action.” Last updated January 27, 2009. Accessed August 2009. (Includes discussion of
capillary action in soil.) URL no longer active.
Smith, S. E. “What is Capillary Action?” Accessed June 2010.
http://www.wisegeek.com/what-is-capillary-action.htm.
Stein, Becky. “Capillary Action.” Last updated August 8, 2009. Chemwiki.ucdavis.edu. Accessed July 2010.
http://chemwiki.ucdavis.edu/Physical_Chemistry/Physical_Properties_of_Matter/Intermolecular_Forces/Cohesive
_And_Adhesive_Forces/Capillary_Action.
Contributors
Jean Stave, Durham Public Schools, North Carolina, Professor Chuan-Hua Chen, Mechanical Engineering and Material
Science, Pratt School of Engineering, Duke University.
Copyright © 2011 by Mechanical Engineering and Material Science, Pratt School of Engineering, Duke University.
This digital library content was developed under an NSF CAREER Award (CBET- 08-46705) and an RET supplement
(CBET-10-09869).
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