There are heaps of videos on the internet showing the movement of food coloring on top of milk when detergent is added. It is a wonderfully simple, colorful and fascinating experiment to do at home.

But…how does it work? How does adding detergent to milk with food coloring on top cause the food coloring to “magically” move?

When detergent is added to milk with food coloring on top it causes two things to happen:

  1. The detergent breaks the surface tension of the milk 
  2. The detergent finds milk components to join up with

These processes create an amazing display of color moving quickly in streaks and swirls. The food coloring is caught up in, and carried along with these two things happening. These two things happen anyway, but the food coloring dramatically shows what is happening. 

How does detergent break the surface tension of the milk?

Milk is mostly water (about 87% water). Surface tension in milk is due to water molecules on the surface of the milk being attracted to each other. Imagine water molecules to be like tiny magnets. When these tiny magnets are all jumbled up together (as molecules of water are) they push and pull each other in all directions. At the surface of the milk the water molecules do not get pushed and pulled in all directions. They are able to arrange themselves to be pulled only sideways and downwards. This surface arrangement is held together more strongly than the movement happening underneath. The strength of this surface arrangement is referred to as surface tension.

When detergent first hits the surface of the milk it is like inserting something that is not magnetic into the surface network of magnets. This insertion breaks the tiny network of magnets and the magnets all pull towards each other, away from the detergent.

Why are water molecules like tiny magnets?

Water molecules are in a tug of war with themselves. Water is made up of oxygen and two hydrogen molecules. Oxygen is a big strong tough molecule and Hydrogen is a much smaller, weaker molecule. Oxygen is winning the tug of war. In winning this tug of water it is pulling electrons (which are negatively charged) closer to itself, in doing so oxygen is becoming slightly negative. Hydrogen, on the other hand, is losing the tug of war, the electrons are being pulled away from it so it is becoming slightly positive. Water molecules are therefore slightly negative on the oxygen side of the molecule and slightly positive on the hydrogen side of the molecule – like a tiny magnet. They are not actually magnetic, it is an electric charge, but magnets are a useful analogy. The attraction of water molecules is called hydrogen bonding.

So that explains the initial movement of the food coloring. It is getting caught up in and pulled along with this initial movement of the tiny magnets (the water molecules). So why does the food coloring keep moving. The dramatic initial burst of movement is over but the swirling starts and keeps going, for quite a while. Why? What is causing this amazing display of color on the surface of the milk?

How and why does the detergent finds milk components to join up with?

The detergent not only breaks the surface tension but it also goes looking for milk components to join up with. The detergent molecule, like all soap, has two ends – an end that wants to join up with water (water-loving, or hydrophillic) and an end that wants to get away from water (water-fearing, or hydrophobic). The hydrophobic end of the detergent molecule rushes off to find a “safe” place to hide. It needs to get away from the water, the “safest” place to hide is with fat molecules, which themselves also don’t like being in water. This rushing around to hide pushes the food colouring molecules around, so you see this explosion of colour, which can go on for some time, until all the detergent molecules have “hidden”, or linked-up with the fat.

All this movement that can be seen on top of the milk is going outwards, away from the detergent, as the milk moves away from source of the detergent it gets replaced by milk from underneath, which in turn gets pushed away by the detergent. A current has been created in the milk. Try doing the experiment by soaking a piece of cotton wool in detergent and placing it in the middle of a shallow plate that you can see through. Then you can view the colour movement from above and below and the same time. What do you see? Fascinating right!

How to do the Magic Milk Experiment

1. Gather your supplies – Plate, Milk, Food Coloring, Dish Soap

  • Clean shallow dish or plate (I have actually done it with a plastic sandpit (minus the sand) at my daughter’s birthday party, with a whole bunch of kids crowded around.)
  • Milk – Whole (Full) or 2% (Light) recommended. But experiment with what you have – try skim, almond, coconut and see which is best!
  • Food coloring
  • Dish soap
  • A small dish for the dish soap
  • Cotton swabs (buds) or toothpicks

2. Fill a clean shallow dish with milk.

3. Drop in some food coloring

You won’t need much, as I am sure you know! Use as many colors as you like.

4. Drop in some detergent and watch the colors spread. 

I like to put a tiny amount on the end of a toothpick then prick the milk in a few places to see collisions of color. I love to do this first with a toothpick, without detergent on it, then again with detergent on it (but so the kids don’t know). Love the looks of amazement as the colors spread.

What do you see?

Color spreading quickly away from the point that the detergent was added.

This is a great activity for a kids party, as it appeals to both the budding scientists and the artists. In a big group, this activity is best done outside, with a big bucket to pour the milk/coloring in when they are done. Most kids want to have another go with different colors, so have the means to wash and dry their plate handy.

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