This is another demonstration that I learned from Ben Sill . As with all his demonstrations, it can be done straight or with some slightly over the top dramatization (there are other conceptually similar versions of this demonstration posted online). The goal of the demonstration is to illustrate how surface tension can keep heavier objects afloat and how surfactants and surface tension gradients can drive a flow.
- Box of ground black pepper or pepper in a pepper grinder
- Overhead projector
- Clear square flat bottomed glass bowl
- Water to pour into the bowl
- A bar of soap in your pocket
- A student volunteer
Equipment minus the water, OHP and student.
- Set up the projector and place the bowl full of water on it
- Sprinkle the ground pepper on the surface
- Ask the student volunteer to divide the peppers in half by running their finger down the middle of the bowl. The pepper should not divide because it will be drawn into the wake behind the student’s finger
- [optional step for the extreme extroverts] tell the class that the student did not do it correctly because they should have said “Divide, O great pepper!” Do the next step with the class repeating this phrase.
- Scrape a little soap under your finger nail while it is still in your pocket. Then run your finger across the top of the water. The pepper flakes should divide. Note that you have to have the soap flake run along the surface of the water. If the soap is submerged the surfactant will not create the surface tension gradient needed to part the water.
When you sprinkle the pepper on the water some of it sinks and some floats. The pepper that floats is held up by curvature in the water surface just like water a strider.
The soap (surfactant) creates a region of low surface tension down the middle of the bowl. This surface tension gradient drives a flow away from the region of low surface tension and divides the pepper flakes. Surface tension driven convection (Marangoni convection) is used by some insects to propel themselves across a water surface.
An index of all the demonstrations posted on this blog can be found here. Don’t forget to follow @nbkaye on twitter for updates to this blog. If you have a demonstration that you use in class that you would like to share on this blog please email me (firstname.lastname@example.org). I also welcome comments (through the comments section or via email) on improving the demonstrations.