Reducing tension

bubbleHave you ever tried to float a paperclip in a bowl of water?  It is quite difficult but possible if you put the paperclip on a piece of tissue paper and carefully place the tissue paper with the paperclip onto the surface of the water; then, using a pencil slowly push the tissue underwater and, with a little bit of luck and practice, the paperclip will be left floating on the surface of the water.  The surface tension of the water counteracts the gravitational force on the paperclip.  This is the same mechanism that allows some insects to ‘skate’ across the surface of ponds.

Detergent is a surfactant which reduces the surface tension of the water.  So, if you drop a little bit into your bowl of water the paperclip will sink because the surface tension is no longer sufficient to support it.

This is not an experiment to demonstrate in class because it is too delicate and too small for students to see but students can do it for themselves at home.  An alternative for demonstrating surface tension effects is to blow bubbles using a detergent solution.  These two ‘Everyday Engineering Examples’ are described in the lesson plan below and you can watch a video clip about it at www.youtube.com/watch?v=BRyQvGEQUt0

5EplanNoF1_fluids&their_properties

See also the Everyday Engineering Examples page on this blog for more lesson plans and more background on Everyday Examples.

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