Brightly coloured liquids spread across the surface of milk by adding soap to break the surface tension.

This is often known as tie-dyed milk because it will end up looking like we have dyed the surface of the milk with some beautiful colours! We will see colours run away repelled by a drop of soap!

Yes, I made a forest of laser cut Christmas trees… I made a whole bunch to send out to local schools but couldn’t get the design quite right so my forest is made of all the prototypes. I don’t learn quickly. It is a large forest.

What you will need

  • A dish, bowl or plastic tray
  • Milk
  • Food colouring
  • Washing-up liquid

How to do it

Try and make sure that the tray you are using is as clean as possible before you start – especially that there are no suds or soap left on it. This will help make the results look better!

Pour milk into the container – it doesn’t need to be a deep layer of milk but it does need to cover the bottom of the container entirely.

Gently drip food colouring into a pattern on the surface. I tend to go near the middle but not exactly in the middle – this helps you to keep your different colours separate from each other. If you drip them on top of each other they will all just mix up.

Put a drop of washing up liquid either on your finger or on a cotton bud and dip it into the middle of the milk…

…and watch the colours explode across the surface of the milk.

Some interesting things happened…

Some food colourings stay put in small droplets and some really spread out on the surface on their own. You want to use ones that stay put for this, really. In my colours, yellow tended to spread out a lot.

Interestingly the yellow food colouring also made little floating droplets on the surface. I’ve heard them called antibubbles…

I ended up with some of my prettiest results just from using the yellow food colouring to make the colours ‘explode’.

What’s happening?

You can get a similar result with just water and pepper.

Water and water-based liquids like milk have surface tension.

This means that all the molecules water are pulling tightly on all the molecules around them and making a big chain of connections. It’s as if there is a crowd of people in a room all holding hands with each other and making links.

Water molecules are really good at pulling each other closely and tightly together but gas molecules in the air are not! These gas molecules don’t want to hold hands with the water and want to stay on their own, running about like crazy and never staying still (I can relate).

Because the gas molecules outside the liquid don’t want to make friends and hold tightly to the water molecules, all the water at the outside of the container – or the people on the edge of the huddling-hand-holders – have fewer hands to hold onto… The molecules at the surface or the people at the edge get a bit lonely.

Washing-up liquid is a type of soap and contains something called a surfactant. They are named so because they are surface active. They act on a liquid surface. The soapy molecules are brilliant because they make really good friends – they can be energetic and play happily with gas molecules, but they can also huddle and hold hands with water molecules. When you add them to the water they spread out all over the surface of the water keeping the huddling water and the hyper air apart. Now that neither the water or the gas see each other, everyone is a lot happier.

So the soap spreads out across the surface, what’s that to do with the colours we see?

As the soap spreads out across the surface it is replacing the water that used to make up that skin on the surface.

Remember the water molecules all pulling really strongly on each other? That’s why it’s called surface tension, because they are all pulling each other like pulling on an elastic band. The soap molecules don’t pull on each other in quite the same way so it’s like letting go of the elastic band – ping! The tension is released and snaps away from wherever you let go of the elastic band.

Since you dropped the soap into the middle, that’s where the tension gets released! The soap hasn’t reached the edges of the tray yet so the water at the surface there is still pulling away and under tension! Those water molecules pull everything along towards the edge and that carries all that colour along with it.

There are some additional effects from the fat in the milk – but I don’t think they are the real driving factor here. Some sites will say that the fat is all that matters because you get different results with different amounts of fat in the milk – which is a fair observation to make. The fat is definitely important but since this effect is mostly seen at the surface – surface tension is the real key player to watch.

Get Involved

This is an easy one and always impresses. Give it a try and snap a pic, share it with everyone with #XPeriMas on twitter!

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