Callum, Danny and James, of Paraparaumu Beach School, asks :-
We put three drops of different food colouring at the corners of a triangle on the surface of a bowl of milk, and then put a drop of detergent in the middle of the triangle. All the food colouring shot out to the side. Why?
Gorden Heeley, a chemist at Victoria University of Wellington, responded.
The experiment you did is all about something called surface tension.
If you look closely at a drop of water on a polished surface you will see that it is shaped like a ball. This is because there is something like a stretchy skin on the water which is pulling all the water molecules into the smallest shape possible, which is of course a sphere (ball). This stretchy skin is called surface tension. Water acts like it has a stretchy skin because the water molecules are strongly attracted to each other so they tend to stick together. Detergent molecules squeeze between water molecules, pushing them apart and reducing the water's surface tension.
Milk, which is mostly water, also has surface tension. When you add the detergent to the milk you reduce the surface tension of the milk at that spot. Since the surface tension there is much weaker than in the rest of the milk, the water molecules elsewhere in the milk pull the water molecules away from the soapy spot. The movement of the food colouring follows the movement of the water molecules.