Agnes Pockels: A Science Super Hero

To celebrate International Day of Women and Girls in Science (February 11th) 2021, Phil has been helping the Global Science Show on twitter to pull together a mini show filled with stories of role models and fantastic female forces in science. Phil chose Agnes Pockels, a self-trained chemist who discovered so much about surface tension. You might have noticed that Phil really likes surface tension from some of the other pages on WMG Experiments!

a cartoon button stuck to a water surface with surface tension

Surface Tension Measurements

You Will Need:

A Pockels' Balance!

I'm working on some plans for a home made version of this that anyone can recreate. Bear with me.

A button or other flat object you can tie some thread to.
Thread
A clean bowl or plate
Water

Recreate it:

Tie your thread onto the button so that you can hold the thread and dangle the button below. You want the button to be as flat as possible so you might take some time to adjust your knots to get it just right.

Clean the bowl or plate of water with soap and water and then keep rinsing it until you're sure that all traces of that soap have been washed away. Rinse it a bit more for good luck. Any soap or grease left on the plate will ruin this!

Fill the plate or bowl up with water and check if there is any froth or foam on the water. If there is, there's still soap in your bowl/plate! Rinse it off and try again.

Take a clean tissue and wipe the button carefully to remove any oil from your fingers. You must not touch the button again after this!

Dangle your button down until it touches the water surface. Now, being really gentle, see if you can lift the button up a tiny bit. If you watch from the side you might notice that you can pull some of the water up with the button until it 'snaps' and the water lets go of your button.

Hopefully it will look something like this:

a short clip showing the surface tension formed between a wire and the water surface. When the wire is raised up the water appears to snap and falls back into the bowl below.

How does it work?

Okay - by now I'm sure you've read some of the other WMG Experiments pages and heard me call water 'sticky'. Well here's the best demonstration of that. The wire is literally sticking to the water surface and we have to peel it away! Once one side of the wire gets too high and the water is being 'stretched' too far, the water falls back into the trough and the wire lifts away easily.

In the video you'll see that the balance is perfectly... well, balanced until we touch the wire to the water. After that, we can add more weight to the bucket before the wire will lift off the water surface again.

Extra weight added to one side means that there is more force trying to tip the balance. We know that if the balance doesn't move, there must be balanced forces acting on either side! So on the side with the bucket we have gravity pulling down.. what do you think is pulling the wire down on the other side? Surface tension! This is the clearest way I know of to show that surface tension is a force and is actually quite considerable!

You can actually measure how strong the surface tension is here. We need to know:

How much weight do we add to the bucket to lift the wire from the water surface?
How long is the distance from the pivot of the balance to the point where the bucket is attached?
How long is the distance from the pivot of the balance to the point where the wire is attached?
How long is the bit of wire that is actually touching the water surface?

You can measure how much weight you need to add to the bucket to make the wire lift off and then do an experiment by changing some variables!

Variables

Do you need more weight to lift the wire out of sugary water than plain water?
What about if you added a little oil to the water?
Do you need more weight to lift a longer wire/bigger button out of the water?