Curricular Unit: Surface Tension

Contributed by: NSF CAREER Award and RET Program, Mechanical Engineering and Material Science, Pratt School of Engineering, Duke University

Four images (clockwise from top left): A water strider insect, a lotus flower with many white petals, apparatus to measure surface tension by how high a liquid climbs a capillary tube, and water droplets on a leaf.
Surface tension can be seen in a variety of places
copyright
Copyright © (left to right) Tim Vickers, Jakub Halun, Michael Krahe, Muhmmad Mahdi Karim, Wikimedia Commons http://commons.wikimedia.org/wiki/File:Water_strider.jpg http://commons.wikimedia.org/wiki/File:20090809_Lotus_flower_2736.jpg http://commons.wikimedia.org/wiki/File:Oberfl%C3%A4chenspannung_Kapillareffekt.jpg http://commons.wikimedia.org/wiki/File:Flower_reflections_in_raindrop.jpg

Summary

Surface tension accounts for many of the interesting properties we associate with water. By learning about surface tension and adhesive forces, students learn why liquid jets of water break into droplets rather than staying in a continuous stream. Through hands-on activities, students learn how the combination of adhesive forces and cohesive forces cause capillary motion. They study different effects of capillary motion and use capillary motion to measure surface tension. Students explore the phenomena of wetting and hydrophobic and hydrophilic surfaces and see how water's behavior changes when a surface is treated with different coatings. A lotus leaf is a natural example of a superhydrophobic surface, with its water-repellent, self-cleaning characteristics. Students examine the lotus effect on natural leaves and human-made superhydrophobic surfaces, and explore how the lotus leaf repels dewy water through vibration. See the Unit Overview section for details on each lesson in this unit.
This engineering curriculum meets Next Generation Science Standards (NGSS).

Engineering Connection

Understanding the effects of surface tension has implications in a wide variety of engineering fields. For example, the functioning of the widely-used inkjet printer depends on the surface tension of the ink turning a jet of liquid into droplets that can be deployed into position. Environmental and chemical engineers study the interactions between surface tension and chemicals as they develop technologies to remove pollutants from our water and air resources. The surface tension of paints and other surface coatings is carefully designed to ensure that the coating spreads easily while maintaining a desired film thickness. Another consequence of surface tension, the wetting or beading of a liquid on a surface, is important in the design of coatings that to improve visibility through vehicle windows by helping water to shed quickly on the outside of windows while avoiding the formation of tiny droplets of condensation that create a "foggy" appearance on the inside of windows. Integrating the scientific understanding of capillarity and wetting, chemical engineers have created modern materials and coatings so that clothing and other surfaces repel water and are self-cleaning.

More Curriculum Like This

Superhydrophobicity — The Lotus Effect

Students are introduced to superhydrophobic surfaces and the "lotus effect." Students learn how plants create and use superhydrophobic surfaces in nature and how engineers have created human-made products that mimic the properties of these natural surfaces.

Wetting and Contact Angle

Students are presented with the concepts of wetting and contact angle. They are also introduced to the distinction between hydrophobic and hydrophilic surfaces. Students observe how different surfaces are used to maintain visibility under different conditions.

High School Lesson
Surfactants: Helping Molecules Get Along

Students learn about the basics of molecules and how they interact with each other. They learn about the idea of polar and non-polar molecules and how they act with other fluids and surfaces. Students acquire a conceptual understanding of surfactant molecules and how they work on a molecular level. ...

Exploring the Lotus Effect

Students test and observe the "self-cleaning" lotus effect using a lotus leaf and cloth treated with a synthetic lotus-like superhydrophobic coating. They also observe the Wenzel and Cassie Baxter wetting states by creating and manipulating condensation droplets on the leaf surface.

High School Activity

Educational Standards

Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.

All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN), a project of D2L (www.achievementstandards.org).

In the ASN, standards are hierarchically structured: first by source; e.g., by state; within source by type; e.g., science or mathematics; within type by subtype, then by grade, etc.

  • Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles. (Grades 9 - 12) Details... View more aligned curriculum... Do you agree with this alignment?
  • Communicate scientific and technical information about why the molecular-level structure is important in the functioning of designed materials. (Grades 9 - 12) Details... View more aligned curriculum... Do you agree with this alignment?
  • Knowledge gained from other fields of study has a direct effect on the development of technological products and systems. (Grades 6 - 8) Details... View more aligned curriculum... Do you agree with this alignment?
Suggest an alignment not listed above

Unit Overview

Lesson 1: The first lesson introduces surface tension and adhesive forces and explores the question of why liquid jets break into water droplets rather than remaining in continuous streams. In the associated activity, students use common household materials to explore surface tension phenomena and surfactants; they determine their own best soap solutions for making bubbles and experiment with additives. Calculations from a math homework explain why soap bubbles form into spheres.

Lesson 2: The second lesson builds on the first by introducing cohesive forces and demonstrating how the combination of adhesive forces and cohesive forces cause capillary motion. By studying the capillary motion of a liquid, its surface tension can be measured. Two activities are associated with this lesson: in the first, students study different effects of capillary motion; in the second, for more advanced students, they use capillary motion to measure the surface tension of water.

Lesson 3: The third lesson introduces the concepts of wetting and hydrophobic and hydrophilic surfaces. These phenomena also depend on the surface tension of the liquid and the strength of a liquid's attraction to the surface. In the associated activity, students explore how water's behavior changes when a surface is treated with different coatings.

Lesson 4: The fourth and final lesson in this unit introduces superhydrophobic surfaces with the lotus leaf being a natural example. Superhydrophobic surfaces are covered with tiny protrusions that trap a thin layer of air between a water droplet and the surface, and the air cushion gives rise to the water-repellent, self-cleaning property of the lotus leaf. In the associated activity, students examine the lotus effect on natural leaves and human-made superhydrophobic surfaces (an example of biomimicry), and explore how the lotus leaf repels dewy water through vibration.

Unit Schedule

Contributors

Jean Stave, Durham Public Schools, NC; Chuan-Hua Chen, Mechanical Engineering and Material Science, Pratt School of Engineering, Duke University; Jonathan Boreyko, Mechanical Engineering and Material Science, Pratt School of Engineering, Duke University

Copyright

© 2013 by Regents of the University of Colorado; original © 2011 Duke University

Supporting Program

NSF CAREER Award and RET Program, Mechanical Engineering and Material Science, Pratt School of Engineering, Duke University

Acknowledgements

This digital library content was developed under an NSF CAREER Award (CBET- 08-46705) and an RET supplement (CBET-10-09869). However, these contents do not necessarily represent the policies of the National Science Foundation, and you should not assume endorsement by the federal government.

Last modified: June 6, 2017

Comments