Super Cool Surface Tension

Creative Commons License photo credit: shioshvili

Did you know that water is sticky?  It may not look sticky, but water molecules love to stick together.  Water molecules pull and tug on each other from all directions.

Look at a glass of water. A special stretchy “skin” forms at the top of a glass of water because the molecules are pulling from the sides and from below, but not from the top.  This “skin” is known as surface tension.  Surface tension allows the water level to get higher than the walls of the glass without spilling!  Let’s put this to the test.

Drinking glass
50-100 pennies
Paper towels

1. Fill a glass to the top with tap water.
2. Set your glass on a paper towel.
3. Carefully add pennies one at a time.  Be sure to keep count. 
4. Observe the water at the top of the cup.  Eventually it will begin to bulge out.
5. Add pennies until the water begins to drip over the edge of the glass.  How many pennies were you able to get into the cup?
6. Now, remove all of the pennies and get a new cup of tap water.  This time add several squirts of dishwashing liquid to the cup and try the activity again.
7. Were you able to add more pennies or did it hold less?  What do you think is happening?  Research and find out!

I can’t find my reading glasses, but I’ll manage.

You’ve probably noticed the magnifying effect of a glass of water or any other clear beverage (the black text to the right of the glass is the same size as the black text behind the glass):

And you probably have some idea that the magnification has to do with the curved shape of the glass and the water it contains: The water in the glass bends light so it appears to us to be coming from an object that is bigger or closer than it really is.

To explore this more, try making differently sized water drops on top of a sheet of waxed paper (the waxed paper helps the water ‘bead up,’ which improves the effect):

You’re aiming for a large drop about 2 centimeters or 1 inch across, and medium and small drops that are, well, smaller.  If you don’t have an eyedropper to help you, you can either pour extremely carefully or dip a pencil or spoon in water and let the water drip off of it.

Look at a page with words through the drops (don’t use your first editions of The Old Man and the Sea or Einstein’s General Theory of Relativity, because the water will eventually seep through the waxed paper and make you very, very sad).  Do you see any differences between the larger and smaller drops?

This looks much clearer if you try it yourself, so go do it! 

You may be thinking “My large drops (possibly puddles) don’t seem to change anything; why do the small drops work so much better?”  To explain this, try looking at your drops from the side (your eyes should be level with the surface of your table:

The shapes are different: The largest drop looks almost flat across the top, while the smallest drop makes a very tidy little dome shape.  Another way to say this is that the smallest drop’s surface is more sharply curved, or is more convex than the larger drops (convex surfaces bulge out, concave surfaces “cave in.” And it turns out that the less convex the surface of the drop, the less it magnifies.  If you want a more in depth explanation with diagrams, check out this site.

Convex and concave lenses are used in all kinds of cool equipment. For more information on lenses and the anatomy of your eyeballs, check out The Anatomy of the Eye.