More surface tension effects
Practical Activity for 14-16
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
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.
The waxed slides are difficult to clean for re-use; best to throw away.
- Clean a microscope slide carefully so that there is no grease or oil on it. Use a strong detergent and rinse well.
- 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.
- Fill a bulb pipette with water and drop pools of water about 1 cm diameter onto each slide. Compare the drops.
- 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.
- Repeat steps 3 and 4 with mercury drops.
- 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 higherbecause the adhesive force between water and wax modifies its angle of contact. The water does not
wetthe 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