A storm in a saucer
Henry is 6 and Charlotte is 4 and today they showed me their experiment called 'A Storm In A Saucer".
We started with a saucer and filled it with milk then added a few drops of blue food colouring which left patches of blue on the plate. Not much happened until we added a few drops of washing up liquid and then suddenly the plate turned into a moving, swirling, dancing scene that look on a life of its own.
Henry said "The top of the milk is tight like a balloon and every time you put the detergent in it, it makes the food colouring rush to the sides."
The science behind this is that milk stays together as one volume of liquid because of a force called surface tension. This acts a bit like a skin and keeps the milk together on the saucer. When you add the washing up liquid, it breaks the surface tension of the milk in that one spot which results in a pull of the surface tension from the milk at the edge of the saucer. This causes the milk in the centre to move to the outside and it takes everything including the blue food colouring with it. The reaction happens until the washing up liquid stops affecting the milk, but it only takes one new drop to start it all over again!