# Surface tension

##### Class practical

Surface tension in a liquid is an intrinsically interesting phenomenon. It also provides evidence in the human-sized world of the unseen inter-particle forces that cause it.

#### Apparatus and materials

For each student or group of students

Wire, bendable

Paper clips, small

Detergent solution

Chalk

T-piece or Y-piece and rubber tubing

#### Procedure

a Make a loop of wire about 5 to 10 centimetres across. Tie two pieces of thread across it fairly loosely.

b Dip the loop into detergent solution to make a 'soap film'.

c Touch the film between the threads with a piece of chalk, to burst the film only between the threads. Watch what happens.

d Make a 'square' frame of wire with three sides. Tie a thread across the open end. Tie a second thread to the centre of the loose thread. Dip the frame into detergent solution to make a 'soap film'. Hold the frame with the centre thread hanging down. Try pulling gently on the hanging thread, and letting go. Watch how it behaves.

e Hang the small paper clip from the hanging thread. Make a new film. See if the film will support the weight of the paper clip.

#### Technical notes

1 These are 'surface tension' effects, which arise from the forces between the particles of the liquid. Particles of the liquid attract each other. Particles below the surface experience forces in all directions. Particles on a surface do not. Unlike in a solid, particles in a liquid respond to the effects of forces by changing their location in the material. Each particle on the surface of a liquid is also attracted by its neighbours, but its neighbours are not distributed all around it being 'below' it in the liquid. Particles on a liquid surface tend not to stay there for very long.

One result is that the surface of a liquid becomes as small as circumstances allow. Small quantities of liquid form spherical drops.

(A flock of penguins make an amusing analogy. On a cold Antarctic night, penguins on the outside of a flock would prefer to be on the inside of the huddle. The result is that the shape of the huddle or flock becomes such as to minimize the size of the perimeter. The shape will change if there is a strong wind from one direction, of course.)

Soap films, have double surfaces. They also tend to become as small as possible. This produces an effective tension in the surface which pulls on the threads and can even balance the weight of a small paper clip.

2 The following are suitable for demonstration.

• Dip two arms of the Y-tube or T-tube into detergent solution. Pinch the rubber tubing of one arm and blow a bubble at the end of the other. Then pinch the tubing of the other arm and blow a second bubble. Allow both lengths of the rubber tubing to be open. Watch what happens. Ask the class to explain it. The tendency of surface tension to reduce bubble size is balanced by the pressure of the air inside.

• Float three matchsticks on water, so that they form a closed triangle. Touch the water between them with the corner of a bar of soap. What happens can be explained in terms of the change in inter-particle forces (and hence in surface tension) that is caused by the presence of soap in the water.
• Pour some molten wax onto a scrap of wood to give it a waterproof surface. Put a drop of water onto the surface. Look at it carefully and then touch it with a matchstick which has been dipped in detergent.

This experiment was safety-checked in October 2004