Capillarity—Measuring Surface Tension
The surface tension of a liquid can be found by measuring how high a liquid climbs a capillary tube.Copyright Michael Krahe, Wikimedia Commons http://commons.wikimedia.org/wiki/File:Oberfl%C3%A4chenspannung_Kapillareffekt.jpg
Students are presented with a short lesson on the difference between cohesive forces (the forces that hold water molecules together and create surface tension) and adhesive forces (the forces that causes water to "stick" to solid surfaces. The interaction between cohesive forces and adhesive forces causes the well-known capillary action. Students also use the associated activities to introduce examples of capillary action found in nature and in our day-to-day lives.
Many industrial processes depend on the accurate measurement of surface tension. When an object is painted or coated, the coating surface tension must be carefully maintained to produce the desired thickness without creating uneven patches. The strength and effectiveness of detergents are also partially determined by surface tension. One accurate method of measuring surface tension is through capillary action. The height water rises in a thin tube is related to the surface tension of the climbing liquid. Besides providing a method for measuring surface tension, capillary action itself has many different applications. Capillary action in part determines the behavior of ground water in the soil, which makes it important to civil and environmental engineers in understanding the stability of buildings and roads as well as the environmental impact of human development. Petroleum engineers use their understanding of capillary action in the extraction of crude oil from its rock reservoirs. And, understanding capillary action in the transport of fluids in animals is important in biomedical engineering.
After this lesson, students should be able to:
- Describe the differences between cohesive and adhesive forces.
- Explain how a meniscus is formed.
- Describe how the combination of adhesive and cohesive forces causes water to rise in a thin tube or other small space (capillary action).
- Explain how capillary action is used.