walking water surface tension experiment

There are a lot of examples of this experiment out there on the internet, so I’m not claiming that this was my idea but, when I tried this out for the first time I was really surprised at how good it ended up looking for the amount of effort I had to put in.

All you’re going to need are some cups, some food colouring, some water and some tissue, paper towel or filter paper.

You will need:

  • Water
  • Cups – an odd number, about twice as many as the number of colours of food colouring you have
  • Food colouring – as many colours as you can find
  • Some kind of absorbent paper – tissue, kitchen roll, filter paper

How to do it:

  1. Arrange your cups in a line
  2. Fill every other cup with water so that you have alternating filled-empty-filled etc.
  3. Add a few drops of food colouring to each of the cups of water
  4. Fold your absorbent paper so that it has a bend in the middle
  5. Place the absorbent paper over the edges of two cups so that one end is in the coloured water and the other end is hanging into the next empty cup
  6. Other than the two cups on the edges, all the cups should have two pieces of paper – one in either direction up and down the line of cups
  7. Wait! The water will be drawn up through the paper and over the bend you folded – it will then fall down through the rest of the paper and drip into the empty cup. Slowly, the empty cups will be filled up with a little coloured water from either side so that you’ll end up with the empty cups being coloured with a mixture of the two neighbouring colours.

How it works:

This all works because of the water being pulled up through the absorbent material. The more absorbent the material you choose, the faster the empty cups will be filled.

So what makes a material absorbent? It all depends on the fact that water molecules and air molecules do not get along at all. This leads to the kinds of effects we’ve seen in other experiments like the Floating Paperclips. Materials like paper are made up of loads of tiny fibres smashed together – so there are a lot of gaps in between those fibres. Water can get into those gaps and effectively, hide away from the air molecules they hate so much. The material that makes up those fibres – cellulose – is hydrophilic. This means that it likes water, so inside the gaps between the cellulose fibres, not only are the water molecules able to avoid the air, they also interact with atoms in the cellulose molecules that like water too. This, all round, makes for some pretty ‘happy’ water molecules. There are plenty of materials that can absorb water well, such as cotton or the fantastically named upsalite which all rely on having lots of tiny gaps for ‘happy’ water.

Once some water gets into the gaps in the fibre, they don’t stay still. The water molecules are constantly moving about and some of them get a little further through the material. Since the water molecules are ‘happy’ inside the material it’s more likely that they will stay inside the material and molecules in the full cup are more likely to join them. The other amazing thing that water can do is to pull more molecules along as it moves – this is known as capillary action. So what ends up happening is that the water molecules inside the material keep on moving higher up through the paper, pulling more water molecules along with them, until they reach the fold right at the top and down the other side – now being helped out by gravity too.

At the end of the material the water molecules gather together and eventually there are more molecules there than can fit into the material and so the water starts to sit on the outside of the material instead. Eventually, there is so much excess water at the end of the material that a droplet starts to form – it will always be a nice, round shape to try and make sure that as much of the water is in the middle of the droplet and not on the outsides – remember that water and air don’t get along! The droplet will get too heavy for the material to hold on to and so it will fall. This gets repeated over and over until there are no empty cups and the water has equalised between all the cups.

How can you take this further?

Usually, I would challenge you to find a way to break the experiment and stop it happening. The problem is that capillary action is something that you might struggle to stop!

You can use different materials – the more absorbent the better (more tiny gaps in the material for the water to get into), or different thicknesses – fold several layers of tissue together and see whether that pulls more water through than a single layer.

You could even try to mix two different colours of food colouring in the full cups and see whether one colour gets pulled through the paper faster than another. This might work best with food colouring mixed with something that struggles to dissolve in water quite as well like ink or an oil-based paint.

What’s the point?

There are a lot of applications for materials that can absorb water well – think of high-end sports tops that can wick away sweat or how you might try to fix a phone after dropping it into your cereal.

Besides that, plants rely on capillary action to get water from the ground up to the tops of their leaves. This is a much more colourful way to see this happening – and it’s also a bit quicker than growing a plant to help with revision.



One response

Leave a Reply

%d bloggers like this: