Wetting and Contact Angle
The interaction of water with the air and the leaf influences the shape of the water droplet.Copyright Muhammad Mahdi Karim, Wikimedia Commons http://commons.wikimedia.org/wiki/File:Flower_reflections_in_raindrop.jpg
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.
Chemical engineers understand that a liquid's tendency to either bead or sheet on a solid surface is determined by the properties of both the surface and the liquid. The resulting hydrophilic ("water loving") and hydrophobic ("water hating") surfaces both have important applications in all types of engineering, including chemical, automotive, nautical, industrial and civil engineering. Applying a hydrophilic anti-fog coating to glass causes any condensation to form into a thin, even layer of water instead of droplets, so the glass remains transparent. Adding hydrophobic rain-repellent glass treatments to windshields causes water to bead and roll off the windshield surface to improve visibility (and safety). Hydrophobic surfaces are also important in protecting surfaces from water damage and stains.
After this lesson, students should be able to:
- Define the term "wetting" and describe how total wetting, partial wetting and no wetting are different.
- Explain how water behaves differently when in contact with hydrophilic and hydrophobic surfaces and how this relates to total, partial and no wetting cases.
- Discuss situations in which applying a hydrophilic or hydrophobic surface would be advantageous.