Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

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.

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

 

Procedure


a Clean a microscope slide carefully so that there is no grease or oil on it. Use a strong detergent and rinse well. 

b 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. 
 
c Fill a bulb pipette with water and drop pools of water about 1 cm diameter onto each slide. Compare the drops. 
 
d 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. 
 
e Repeat steps c and d with mercury drops. 

Slide proofs

Teaching notes


1 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 

2 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. 
 
3 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. 
 
4 Mercury has a greater surface tension, so larger drops will maintain their spherical shape. 
 
5 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