Tension Racers!
Students see how different levels of surface tension affect water's ability to move. Teams "race" water droplets down tracks made of different materials, making measurements, collecting data, making calculations, graphing results and comparing to their predictions and the properties of each surface, determining which surface exhibits the highest (or lowest) level of surface tension with water. They apply their results to make engineering recommendations for real-world applications.
Figure 1. General activity setup to race water droplets.Copyright 2010 Samuel DuPont and Ryan Cates. STARS – University of South Florida
The material properties of surfaces are important in engineering design and innovation. Being able to create or treat surfaces on which water does not "stick" has been given much attention in engineering. For example, chemical engineers continually design more effective glass coatings that cause water to easily roll off, preventing a build-up of water on surfaces such as car windshields. Doing this requires an understanding of the concept of surface tension and how water behaves on different surfaces.
After this activity, students should be able to:
- Identify the differences in the interaction between water molecules and various surfaces (water drops form different shapes on different surfaces).
- Establish a connection between the surface tension of a surface and the shape a water drop forms on that surface and the speed with which the water moves across the surface.
- Identify that surface tension is a force and that it acts to pull water molecules together to form a sphere unless another surface pulls on or attracts the water molecules causing the sphere to deform.
- Identify that the amount of deformation of the water sphere is related to how much the water is attracted to or repelled by the surface.