Science corner

12th September 2003 at 01:00
Ray Oliver breaks the surface tension with some simple experiments

On Sunday afternoons, just after the ritual cleaning and waxing of the car is complete, disaster strikes - it starts to rain. The scientific car owner is philosophical and notes the interesting behaviour of water droplets on the shiny surface. The drops refuse to wet the surface but hang around as separate beads. This suggests that water has some kind of elastic skin holding it together. This property is known as surface tension. The water molecules attract each other and form a shape with the smallest possible surface area, a sphere. There are lots of ways that children can investigate surface tension.

Place some waxed paper in a tray and add blobs of water from a dropper.

Rock the tray gently to bring two blobs together. They will coalesce as the water molecules attract each other.

There is a much more beautiful way to illustrate surface tension effects.

It is like watching a film of a tap dripping in slow motion. Fill a tall narrow jar with cooking oil. Since water and oil are immiscible and water is more dense, water will sink through oil. Set up a container of water fitted with a tap, or a plastic tube with a clip.

Allow water to drip very slowly into the oil. The end of the water tube must dip below the oil surface. You will see each drop form, then the neck will stretch slowly and a single drop will detach itself and dive. Often the drops bounce as they hit the base.

Surface tension prevents house dust sinking in clean water. If you stir a teaspoonful of dust with water, most of it stays dry and floats. Now dip a pencil tip into washing-up liquid. Gently touch the centre of the dusty water. The dust will fly away to the edge and sink. Detergents lower the surface tension of water.

Ray Oliver teaches science at St Albans Girls' School, Hertfordshire

Sophie Duncan is back in Science Corner on September 19

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