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Psychedelic Milk

Public summary: 

A little milk, a little food dye, a little washing-up liquid... and an amazing display of colour. See the stunning swirling effects from disrupting the surface tension.

See the amazing patterns made by food colouring and detergent in milk.
Useful information
Kit List: 

- Some milk (usually have UHT in stores)
- A few colours of food colouring
- A few drops of washing up liquid
- A fairly flat bottomed bowl

Packing Away: 

Dispose sensibly of milk etc, wash up!
The food colouring usually lives in Misc

Frequency of use: 

Taken from Dave's Naked Scientists explanation:

In a nutshell
Investigate how reducing surface tension can lead to pretty patterns that move in milk using food colouring and washing up liquid.

How to set up the experiment
1) Add about 1cm depth of milk to the bottom of your bowl.
2) Pour a few drops of each colour of food colouring onto different places on the surface of the milk (be sparing)
3) Add a drop of washing up liquid somewhere in the bowl.
4) The food colouring moves! You should find that at first the food colouring moves away from where you added the washing up liquid, and then it starts welling up from below the surface of the milk, forming beautiful patterns.
5) After a minute or so and everything has stopped moving, add another somewhere else.

What you need to know during the event
1) Milk is mostly water, and water has a property called surface tension. This is because all the water molecules are strongly attracted to other water molecules, but not to air. Therefore the water molecules try to make the surface (the interface between the water and air) as small as possible. This is why raindrops are approximately spherical - the shape with the minimal surface for its volume.

2) The food colouring is less dense than the milk so it floats on the surface. This is because the milk has lots of substances dissolved in it such as calcium, making it more dense than the food colouring (which is almost entirely water).

3) Washing up liquid is designed to reduce the surface tension so water can dissolve fats and grease. This means that where you add the washing up liquid, the surface tension is much weaker than everywhere else. The surfactant spreads across the surface away from the drop, making the rest of the surface shrink.

4) The food colouring is forced downwards and there is a current below the surface flowing back towards the washing up liquid pulling the food colouring along. It then floats back up to the surface producing beautiful patterns.

Want to know more?
A washing up liquid molecule is made up of a water loving head and a water hating tail, so when you add it to water the molecules arrange themselves over the surface - head inwards. The water is strongly attracted to the heads of these molecules, so is now stops trying to reduce its surface area, and the surface tension is far weaker.

The bubbles in washing up liquid are not originally there - they get added because people associate bubbles with cleanliness...

Risk Assessment
Date risk assesment last checked: 
Thu, 30/01/2020
Risk assesment checked by: 
Conor Cafolla
Date risk assesment double checked: 
Fri, 31/01/2020
Risk assesment double-checked by: 
Polly Hooton
Risk Assessment: 
Hazard Risk Affected Person(s) Likelihood Severity Overall Mitigation Likelihood Severity Overall
Surfactant (I) Could get in eyes and result in irritation. All 4 2 8 If children get their fingers in the washing up liquid, tell them not to put their fingers near their eyes and ensure they rinse it off.
If washing up liquid gets into an eye, demonstrator must call a first aider and may perform an eye wash if trained and confident to do so.
2 2 4
Surfactant (II) Could be harmful if ingested All, particularly small children who try to ingest things 3 3 9 Use a relatively harmless surfactant (standard washing up liquid) and watch the kids so they don't try to lick their fingers or drink any part of the experiment.
In the event of an incident, call a first aider.
2 2 4
Spilled milk (or water) Slip hazard. All 4 3 12 All spills should be cleared up immediately. Call first aider in case of injury. Don’t cry over it. 1 3 3
Publicity photo: