Milky surface tension experiment

This experiment is another kitchen cupboard favourite that we learned how to do after a lovely friend shared a video of it on my FaceBook wall.  All you need is milk, food colourings, a container and some liquid soap.

Put some full fat milk into your container, add a few drops of food colourings to make little blobs of it floating in the milk- you may want to experiment with what colouring works best as we found our blue sank and red floated.  Add a few drops of liquid soap, e.g. washing up liquid, on to the blobs of food colouring and your little ones will be amazed as straight away the colouring starts swirling out into the milk.

 The original video that was shared with me didn't have an explanation, but if your little ones are looking for an explanation this is what I think is happening: for very little ones you can tell them that the soap is helping the milk and the colours to mix together.  For older children you can talk about surface tension.  Surface tension is the ability of a liquid to resist an external force - a really good example that you can observe this effect with is to find video footage on the internet of a pond skater walking on water.  The effect is caused by the cohesive properties of molecules in the liquid - the ability of the bits to stick together.  It is responsible for lots of the properties of liquids, for example the way rain falls as drops instead of just remaining in the atmosphere as separate water molecules.

 This experiment works best with full fat milk I think because the fat molecules in the milk are evenly distributed  and when you add the food colouring to the milk the fats prevent the colouring from diffusing (mixing) quickly into the liquid as would happen if you dripped colourings into plain water.  When you add soap I think two main things happen, firstly the soap is a surfactant so it breaks up the bonds between the fat and water in the milk, plus it disrupts the surface tension of the water in the milk - it makes the sticky molecules come unstuck, both of which allows the colours to spread out.  Even more exciting, this is not now a gradual dilution that you get in plain water, the colours shoot around the dish in waves as the colours are driven by movements in the milk as the stronger surface tension areas pull away from the soapy areas where the surface tension is weaker.

After we played with the original set up a few times by replacing the ingredients with a fresh set up in a cleaned dish, we then brought in the idea of scientific method by keeping some parts of our experiment the same and just changing one part - on this day we tried using a different type of liquid soap - hand soap - to see if one soap made the colour mix in faster.  We used simple equipment to measure out our quantities to keep them standard - medicine spoons for the soap, medicine syringes for the colouring and glasses for the milk.  The boys then added the ingredients of their experiment at the same time and watched to see what happened - in our basic set up we didn't see much difference.  I thought there might be because our hand soap was a standard brand while the washing up liquid is an eco brand so I expected it to be less strong of a surfactant.  As it happened it looks like it was just as effective at breaking up the fat molecules and surface tension as the regular soap.