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Surface Tension and Pretty Patterns

  • a shallow dish
  • different coloured food colourings
  • milk
  • dishwashing detergent

Tip milk into a large dish, such as a roasting pan. Carefully drop evenly spaced drops of food colouring in the milk. Make sure you rinse the dropper in some water before you add the next drop, and don't bump the dish. Then drip a little detergent in the centre of the coloured drop.

Watch what happens. Can you explain why?

Try repeating the procedure, but add the detergent to the side of the dish and carefully tip it so that it moves towards the centre.

Explanation

All the colours begin to move -- the one that moves first was the colour that first came into contact with the detergent. The milk behaves as though it has got a skin or film across its surface. The skin or film is caused because the molecules of milk at the surface are more strongly linked than the ones below. This is called surface tension; the strength of which is determined by the strength of the attraction between the molecules of food colouring and milk.

The molecules of milk pulled with an even force in all directions except for upwards because there were no milk molecules above. When the detergent was added, the detergent molecules weakened the surface tension of the milk and allowed the food colouring molecules to move more freely.

Detergent consists of molecules that are partially hydrophobic and hydrophilic. The hydrophilic, or water-liking, part of the molecules are attracted to the water component of the milk, while the hydrophobic, or water-hating, part attempts to get away from the water, forcing the detergent to spread out in an even, thin layer on the surface of the milk. (This process works just as well in water, but the milk provides a useful background against which to see clearly the movement of the food colouring as the detergent spreads out.)

Greg Walker, NZSM

Greg Walker, NZSM