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Hydrogen Bonding

Introduction to the electrostatic attraction between polar molecules.

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Walk on Water

Walk on Water


Credit: felixtsao
Source: http://www.flickr.com/photos/felixtsao/6860628505/
License: CC BY-NC 3.0

This insect is called a water strider. That’s because it really can walk on water. How can it perform this incredible feat? The answer has to do with the nature of water itself.

Amazing But True!

  • Did you ever notice that if you completely fill a glass with water, you can actually cause the water to bulge above the top of the glass without the water dribbling down over the sides? The reason is that water molecules “want” to cling to each other. 
  • Water molecules have polar bonds, which gives the molecules positive and negative ends. Opposite charges attract, so weak bonds, called hydrogen bonds, form between the oppositely charged ends of adjacent water molecules. 
  • Where water meets air at the surface, there are fewer water molecules to bond together. This produces stronger bonds between those water molecules that actually do come into contact with one another. The result is a top layer of strongly bonded water molecules, as shown in the diagram below. This is known as surface tension. 
  • Water has greater surface tension than any other liquid except for mercury.

Credit: USGS
Source: USGS, publicdomain, http://ga.water.usgs.gov/edu/surface-tension.html
License: CC BY-NC 3.0

  • Inspired by the water strider, researchers at Carnegie Mellon University created a tiny robot that was able to walk on water like the insect! 
  • A prototype of their robotic water walker is pictured below. Does this robot have any real-world applications? The inventors have thought of several potential uses. For example, they think it could be given water pollution detectors and used to monitor water quality. 
  • Listen to one of the inventors discussing the robot and its potential uses in the video: http://www.sciencedaily.com/videos/2006/0710-robot_walks_on_water.htm

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With the links below, learn more about surface tension and the robotic water strider. Then answer the following questions.

  1. A paperclip has greater density than water, but it can be made to float on water like a water strider. Follow the instructions in the PBS link above and see if you can achieve this feat. Then explain why it occurs.
  2. A water strider insect is extremely light in weight. In the video above, how were the researchers able to make their robot very light in weight? What material did they use?
  3. Besides monitoring water quality, what other real-world applications of the water-strider robots have been suggested by the researchers in the video?
  4. The researchers in the video have now mimicked the water strider insect's ability to jump on the water. To make a jumping robot, what problem did they have to overcome, and how did they do it? Why might the ability to jump be useful for the robot?
  5. Water's surface tension is good for water striders, but not so good for washing clothes. How do soaps and detergents affect the surface tension of water? How does this help them get things clean?

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Image Attributions

  1. [1]^ Credit: felixtsao; Source: http://www.flickr.com/photos/felixtsao/6860628505/; License: CC BY-NC 3.0
  2. [2]^ Credit: USGS; Source: USGS, publicdomain, http://ga.water.usgs.gov/edu/surface-tension.html; License: CC BY-NC 3.0

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