Surface Tension Experiments

Water Hill

Take a glass which is filled to the rim with water and slowly raise the water level by either carefully dropping coins into the glass or filling it with water from a teaspoon. One can observe that the glass does not run over when the surface reaches the rim. This is because surface tension, acting like an elastic skin, tries to hold the surface together. But, when a drop of liquid detergent falls and touches the surface of the water hill, the glass immediately runs over. This results because the surfactants in the washing liquid strongly decrease the surface tension.

The Strength of Soap

A ring on which a loop of thread is fastened is dipped into a soap solution and then taken out. In the ring a soap skin develops. Since there is a film of soap within and outside the loop of thread the surface tension does not cause any tension on the thread and therefore the loop hangs loosely. But, if one pierces the soap skin inside the loop of thread causing it to burst, the sum of forces is no longer zero. because the surface of the soap skin wants to minimize, the thread is pulled outwards causing it to form a ring.

Swimming Iron

When a razor blade is carefully and evenly laid on the surface of water it swims because the surface tension of the water prevents it from sinking. When a drop of liquid detergent is added to the water the razor blade sinks immediately as the surfactants in the washing liquid strongly reduce the surface tension.

Displacement

Pepper is sprinkled into a container of water. The pepper floats evenly separated on the surface of the water. When liquid detergent is dropped into the water the surface tension at this place decreases. The surface wants to expand and the pepper will be pushed to the rim.