Planets and Pancakes…explained

Let’s find out about the science behind the milk planets activity.

The Science in a Sentence

Food colour helps us see as the fat droplets in milk break up when detergent is added.

The Longer Story

Milk is made up of water, vitamins, minerals, proteins and fat.

Water has a high surface tension. This means that the molecules in water are attracted to each other, but not to the air above the surface. It is a bit like having a thin stretchy film on the surface of the water. This is why water skaters can “walk on water” without sinking.
You can learn more about surface tension in this amazingly beautiful video from BBC Earth Lab. When you first add the food colouring drops, they may float or sink (depending on the brand) but will not spread much.

What is Surface Tension? | Richard Hammond’s Invisible Worlds | BBC Earth Lab

Washing-up liquid contains detergent. When this is added to the milk, it decreases the surface tension. That means that the water molecules are not as strongly connected to each other as before – which allows the colour to mix in.

Detergents do this because they contain long molecules called surfactants, which have a head and a tail. The heads love water (hydrophilic); the tails hate water (hydrophobic).

When added to water, surfactant molecules arrange themselves into supermolecules, called micelles, with the water-loving heads on the outside and the water-hating tails pointing into the centre, hiding from the water. Imagine tadpoles doing synchronised swimming in a ball, their tails all pointing into the middle…or for a more technically accurate description and a nice diagram, check out Science on the Shelves.

The fat molecules in milk are also hydrophobic (water hating) so do not mix with water. They are suspended in the milk as tiny droplets. When the detergent is added to the milk, the fat molecules are attracted to the surfactant tails (united by a mutual dislike of water). This breaks up the fat droplets and creates the swirling patterns we see in the milk.

In summary

  • Milk contains water and fat
  • Adding detergent decreases the surface tension
  • Surfactant heads attract water
  • Surfactant tails attract fat
  • This breaks up the fat droplets
  • The food colour helps us to see the movement

Learn more about water in this BBC Bitesize article or find out about surface tension and hydrogen bonding in water with the Kahn Academy.

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