January Moment of Science – Drops of Water on a Penny

What is the shape of a raindrop? Most people imagine a raindrop falling from the sky takes the form of a teardrop and looks something like this:

raindroppng

But, in reality, raindrops start off as the shape of a ball.  As they travel downward, due to air pressure, they take on the shape of a hamburger bun: round on the top and flat on the bottom, eventually splitting apart to become small ball shapes once again.

raindropsfallling

Water forms the teardrop shape when it is falling out of a faucet or dropper, like in this activity. As the water builds up in the droplet, it gets heavier and heavier, causing it to dangle down while it is still clinging to the rest of the water inside the dropper. This is when we see the teardrop shape. Eventually the water drop becomes so heavy it breaks off from the rest of the water, forming a ball (just like a raindrop!).

PennyPNGFirst, gather your materials:
– Small porcelain or foam plate
– Cup filled with water
– Full-length, non-flex drinking straw
– Copper penny
– Paper towels

Then, follow these instructions:
1. Bend over the top one-third of the straw. Keep the bent part of the straw squeezed closed as you dip the open end of the straw into the cup of water. Slightly relax your fingers so the straw fills with some water. Without squeezing the straw, pull the entire length of straw out of the water. Then squeeze the bent section to create uniform drops of water. This will be your dropper.
2. Place the penny in the center of the plate.
3. Use the dropper to add one drop of water at a time on top of the penny. Be careful not to touch the penny or the surface of the water! Count how many drops you can add before the water spills over.
4. Note the nature of the water’s surface on the penny when it has a lot of water on it – how would you describe this shape?
5. When you think the water is going to burst off the penny, touch it with the tip of the dropper and see what happens!

What is happening? Water can be piled up on the penny due to its cohesiveness, or attraction between the water molecules. The force of cohesion is strongest across the surface (“skin”) of the water. That is why it is called “surface tension.” High surface tension of water allows it to be piled high on a container, even a glass or a bowl! When the surface of this piled high water is touched, it breaks the cohesive and somewhat rigid “skin” of the water mass and it flows out of the break.