Lesson Wetting and Contact Angle

Learning Objectives

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.

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.

NGSS: Next Generation Science Standards - Science

International Technology and Engineering Educators Association - Technology

State Standards

Introduction/Motivation

(Show Figure 1 to the class, either by projection or by passing around a printout. While students are looking at it, ask them questions about previous surface tension lessons and activities, such as the following:)

• What makes water stick to itself and form bubbles and droplets? (Answer: Cohesive forces—the attraction of liquid particles to one another.)
• How does a water strider stay above water? (Answer: Surface tension creates a slight "skin" of more densely packed water molecules. By using a surface on its legs that repels water, the water strider can walk on this "skin" of water without breaking through.)
• Why do water droplets stick to things? (Answer: Adhesive forces—the attraction of liquid particles to other objects.)
• What causes water to be absorbed by a paper towel or move up a capillary tube? (Answer: Capillary action—a combination of cohesive and adhesive forces.)

Look closely at this photograph. What is happening to the rain on the windshield? In order to see better through the windshield, how would you rather the water acted? Do you want the water to stick to the surface or roll off of it? What could you do to make visibility better? (Students may have their own experiences driving in the rain; as time permits, let students share and discuss their experiences. Give students time to think about their answers and make their best educated guesses using what they know from personal experiences.)

Look at the photograph again. What is causing the window to fog up? How does this affect visibility? Would increasing or decreasing the attraction of the water for the glass improve visibility? What could you do to make visibility better?

What causes the "fogging" effect? Both atmospheric fog and "window" fog are made of tiny droplets of water, but they are not the same thing. In both cases, the tiny droplets scatter light in all directions rather than allowing light to pass straight through. This is why it is not helpful to use your high beams in a fog — because the light gets scattered back in a heavy fog when it wouldn't on a clear night. Have any of you experienced this kind of fogging? (Students may relate stories from driving, as well as glasses and goggles fogging up. You may want to remind students that condensation forms because of a temperature difference between a surface and air.)

The fog on the inside of a window is a comparatively small amount of water. An anti-fog coating causes the water to form a layer of water rather than droplets, resulting in a very thin layer of water that is easy to see through. Outside the car window, there is a lot more water to contend with. Even if you could apply a coating to the outside of the window that caused the water to form a layer of water rather than drops, the water layer would accumulate very quickly and the water sheet would soon break up and reduce visibility. In this case, then, it is much more effective to apply a coating that helps remove the water as quickly as possible.

That's where contact angles come into play. A contact angle is a liquid's tendency to either bead or sheet on a solid surface and is determined by the properties of both the surface and the liquid. A hydrophobic, or "water hating" surface, causes water to form droplets on the surface and easily leave the surface. Adding hydrophobic rain-repellent glass treatments to windshields causes water to bead and roll off the surface of the windshield and make driving less treacherous during rainy or snowy conditions.

By contrast, a hydrophilic, or "water loving" surface, causes water to spread out and cover a surface rather than bead. These types of surfaces are especially useful to avoid loss of visibility due to condensation. As we just discussed, water condensates on glass and causes it to become "foggy" because the tiny water droplets formed on the surface scatter light. When a hydrophilic anti-fog coating is applied to the glass, however, the condensation forms a thin, even layer of water instead of the droplets and the glass remains transparent.

Both of these types of surfaces have important applications in automotive and nautical engineering. These treatments may also be important to engineers when considering visibility through glass windows and partitions in buildings. Hydrophobic surfaces are also important in protecting surfaces from water damage and stains.

Lesson Background and Concepts for Teachers

When a liquid drop is placed onto a solid surface, its behavior depends on the adhesive forces between the liquid and the surface (see Figure 2).

If the adhesive forces are attractive (the liquid is attracted to the solid surface), the liquid drop is pulled toward the surface and spreads along the surface. This type of surface is called "hydrophilic," meaning water loving. If the adhesive forces are repellent (the liquid is repelled by the solid surface), the liquid drop minimizes its contact with the surface and is said to "bead." This type of surface is called "hydrophobic."

The photographs in Figure 3 show examples of water on hydrophilic and hydrophobic surfaces. The surface on the left is hydrophilic, so the water droplets spread out to increase the area of the solid surface in which they are in contact. The surface on the right is hydrophobic, so the water beads up in order to decrease the area of the solid surface in which each bead is in contact.

Hydrophobic materials and coatings are used to make water-repellant clothing and backpacks. The inside of milk cartons is coated with wax, a hydrophobic material, to make them waterproof. Wax is also applied to cars and other vehicles to protect them from the weather. Chemical engineers incorporate hydrophobic elements into outdoor paints and stains to protect wood and other building materials from the elements.

Adding "wetting" agents to lower the contact angle and allow liquids to spread are also useful in many areas. Wetting agents increase the ability of paints and other coatings to spread and penetrate surfaces. They improve the surface contact, and therefore improve adhesion, of cements and glues. Anti-fogging coatings are added to camera lenses and diving masks so that condensation forms a thin, transparent sheet of water rather than many small droplets that impede vision.

The strength of the attractive or repellant force is closely related to the "contact angle" between the water drop and the surface (see Figure 4). On a hydrophilic surface, the contact angle will be less than 90°; the water drop tends to spread out and "wet" the surface. On the other hand, if the surface is hydrophobic, the contact angle will be greater than 90°, and instead, the water drop tends to bead up on the surface.

Associated Activities

• Investigating Contact Angle - Students observe the behavior of water when placed on 3-5 different hydrophilic and hydrophobic surfaces. They determine which coatings are best to cause a surface to shed water quickly or to reduce the "fogging" caused by condensation.

Assessment

Pre-Lesson Assessment

Opening Questions: On a note card or in small groups, have students brainstorm to come up with plausible answers to the following questions. Ask for the most in-depth answers they can provide.

• Why does your mirror steam up when you take a shower? (Answer: As water condenses on the mirror, it forms many small droplets that scatter the light and "fog" the glass.)
• Why do some drivers wax their cars? (Answer: At least one reason is to repel water so that rain runs off the surface.)

Post-Introduction Assessment

Water Droplet ID: Find many photographs from Google Images or another source that show water droplets on various surfaces. By looking closely at the images, have students identify the drops as hydrophilic (total wetting or drops with small contact angles) or hydrophobic (no wetting or drops with large [> 90°] contact angles). As a class, discuss student answers. This activity helps to prepare students to identify the surfaces in the associated activity lab as hydrophilic or hydrophobic.

Lesson Summary Assessment

Three-Minute Writing: Hand each student a note card. Instruct them to write down — in three minutes — everything they learned in today's lesson. It doesn't matter if the information is out of order. Small diagrams or drawings are fine. Encourage students to try to fill the note card front and back in the time given. Require that they do this from memory — they may not refer to their notes.

References

Copyright

Contributors

Supporting Program

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.

Special acknowledgements to Jonathan Boreyko, Mechanical Engineering and Material Science, Pratt School of Engineering, Duke University.