Fami
Magic milk: colours on the run
Drops of colour on milk and a touch of soap: the colours rush away in swirls.
Materials
- 150 mlWhole milkor Single cream thinned with water (even more striking)
- 1Dinner plate
- 4 coloursFood colouring in 4 colours
- 1 dropWashing-up liquid
- 1Cotton budor The tip of a toothpick
Step by step
- 1
Cover the bottom of the plate with 150 ml of milk
Whole milk: the fat is the key
- 2
Put 1 drop of each colour in the middle
Close together but not touching
- 3
Dip the tip of a cotton bud in soap
One drop is enough
- 4
Touch the middle of the milk and hold still
Look from above: the colours rush away
Result
The colours run away!
Why does it happen?
The surface of the milk is tight, like a skin. The soap loosens it right where you touch, and the rest of the surface, still tight, pulls outwards and drags the colours with it. The soap also chases the fat in the milk and makes currents, which is why the colours keep swirling for a while.
For parents
The science, so you can answer
Soap breaks the milk's "skin" (surface tension) and the surface pulls the colours outwards.
The particles in a liquid attract each other, and at the surface that pull makes a kind of tight skin: surface tension. Soap weakens it where it touches. Because the skin around it is still tight, it pulls outwards, like a stretched rubber band let go on one side, dragging the colouring floating on top. Then the fat comes in: soap molecules have one end that sticks to water and another that sticks to fat, and as they go after the tiny fat droplets in the milk they move the liquid and make swirls. With skimmed milk almost nothing happens, because it has hardly any fat.
What they will ask you
«Why don't the colours mix by themselves?»
They float on top of the milk and the surface is still. Until the soap moves it, each drop stays put.
«What does the soap do?»
It loosens the milk's skin right where it touches. The rest pulls outwards and takes the colours with it, like letting go of a stretched rubber band.
«Why doesn't it work as well with water?»
With water the colours move away too, but they mix straight away. The fat in milk keeps the soap moving everything for a while.
«Why does it stop in the end?»
The soap has spread over the whole surface, so there's no difference left to pull. Dip the bud again and touch another spot.
In real life: Soap cleans grease off plates for the same reason: one end of its molecule grabs fat and the other grabs water. And pond skaters walk on water thanks to that "skin".
Animated step by step
Follow it with your phone on the table: it pauses at the end of each step.