Getting Under Water’s Skin
There have already been a couple of experiments in this series that rely on surface tension – the upside down jar, the Marangoni boats, the tie-dyed milk, or some older experiments like the Cheerio effect for examples. After a conversation with someone today who has been reading these experiments I remembered the reason that I have always loved these experiments with water and how they exploit surface tension.
We use water all the time, every day. It’s so normal that it is boring and we take it totally for granted. And yet we can mess about with it and make it do some really crazy things.
Today, I remembered that we shouldn’t take water for granted – this does mean that we should remember that some people in the world don’t have access to clean running water whenever they like and we are lucky that we do. Today, then, I’m going to give you a couple of simple tricks you can do with water – just plain water – to remind us how awesome it is that we have it literally on tap.
The Shape of Water
Water will try to form a sphere whenever possible, because water molecules can pull on each other really tightly in a massive hug, so they like to have other water molecules around them. The sphere puts the maximum amount of water in the middle of the shape and the least water on the edges. If you can get into space you will see nice round water droplets but if not, and you’re stuck on Earth with me, you’ll see teardrop shapes when water drips and drops. The teardrop forms when the droplet tries to form a sphere but it gets stretched out as gravity pulls it downwards.
Once water droplets land on a surface, what shape do they flop into?
It depends on what the surface is! We can make water go really flat and spread out over the surface or we can make water sit high up on the surface in really round balls.
Drop a few drops of water onto a flat piece of wax – something like the underside of a tealight. Then drop the same number of drops of water onto some aluminium foil. The drops on the tealight will form into a round ball while the drops on the foil will spread right out into a flat puddle!
How many drops fit on a coin?
Drip a few drops of water onto a coin. It will probably look covered in water already so ask yourself how many more drops you think you can add before any water spills off the coin. Test it out…
…I bet it was more than you thought.
This time try it again and stop a few drops short of when it spilled before. Now take a toothpick and dip one end in some washing-up liquid. Touch it to the bulging water on the coin. Did it suddenly spill? The soap you added lowered the surface tension – how tightly the water molecules at the surface are pulling on each other and without this tension force, the stretched out, curved surface you have made of your water drops collapses and spills out.
Does metal float?
Find something metal and drop it into water. Will it float? Do you think you could ever get a piece of metal to float?
Try a paperclip. If you drop it onto the surface – it will float. If we place it really really gently onto the water surface you will be able to get it to float!
Here are two tricks to do this:
The tissue trick
Place a piece of tissue onto the water surface. Gently place the paperclip onto the tissue. Using something clean and pointy like tweezers or a toothpick, push the tissue underwater and then carefully move it out to the side away from your paperclip. The clip should still be floating!
Making a tool
You can bend a paperclip to make a little shelf.
Place a second paperclip on this shelf and then lower your new tool through the water surface so that your unbent paperclip is laid gently and perfectly flat on the water surface. Keep lowering your tool until it is clear of your unbent clip and then move the tool away to the side and out. Your paperclip should stay floating!
You need to use a really clean container for this. Any grease at all will ruin your chances of getting anything to float because that grease lowers the surface tension. I tend to find it is way easier to get a clean enough surface if I use a glass or ceramic container instead of a plastic one. Do not put your finger in the water. There is oil and dirt and grease on your skin that will lower the surface tension.
Can we be a bit ridiculous about this? If you can get a piece of metal to sit flat on the surface and place it gently enough you should be able to get a metal ruler to float too! I really hope you try these surface tension experiments at home.
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