More surface tension effects

Demonstration

It is quick and simple to demonstrate that surface tension gives droplets a spherical shape.

Apparatus and Materials

• Microscope slides, 2
• Wetting agent (or liquid detergent)
• Bulb pipette
• Paraffin wax
• Matchstick
• Mercury

Health & Safety and Technical Notes

The mercury experiment is best done as a teacher demonstration using a flexcam and screen, since working in a mercury tray obstructs the students' view. A mercury spill-kit should be to hand.

Read our standard health & safety guidance

The waxed slides are difficult to clean for re-use; best to throw away.

Procedure

1. Clean a microscope slide carefully so that there is no grease or oil on it. Use a strong detergent and rinse well.
2. Coat a second microscope slide with paraffin wax by dipping it into molten wax (e.g. 150 g of clean paraffin wax in an old saucepan, heated until it is very hot) or brushing molten wax on with a clean, cheap paint brush.
3. Fill a bulb pipette with water and drop pools of water about 1 cm diameter onto each slide. Compare the drops.
4. Dip a match stick into a wetting agent such as Manoxel-OT or liquid detergent (not as good) and touch the drop on the waxed slide.
5. Repeat steps 3 and 4 with mercury drops.

Teaching Notes

• Students should note that small water drops coming from the dropper are spherical. A sphere gives the smallest surface area for a given volume. When drops become larger, gravity deforms their spherical shape
• When the drop of water falls onto the clean glass slide, it forms a circular patch of water. Surface tension pulls it into a circular shape but gravity pulls it flatter. Surface tension is a cohesive force resulting from attraction between molecules in the liquid.
• A drop of water on the waxed slide stands higher because the adhesive force between water and wax modifies its angle of contact. The water does not wet the wax. A little wetting agent added to the water reduces its surface tension and the drop collapses back onto the waxed slide, resembling the patch of water on the clean slide.
• 'Waxing' the surface of a material is the basis of waterproofing it.
• Mercury has a greater surface tension, so larger drops will maintain their spherical shape.
• An additional demonstration: Make a tray from perforated zinc or other metal. Dip it into molten candle wax. Will it float or hold water?

This experiment was safety-checked in January 2005