A very useful two-dimensional model of the three-dimensional
crystal can be made with a bubble raft. The basic apparatus is shown in Figure
1(a).
A petri dish is partly filled with a bubble solution of 1 part 'Teepol', S parts
glycerol and 32 parts water by volume. Using the 25-gauge hypodermic syringe connected to
the gas tap a raft of bubbles may be blown. The appearance will be similar to that shown in
Figure 2(b). You will notice how all the bubbles pack together lust like atoms in a crystal. The
surface tension between the bubbles pulls them together, and the pressure inside them
prevents them from getting too close.
These bubble rafts behave like real solids: if
they are compressed slightly they will return to their original state but if too great a force is
applied they will rupture, or one plane of bubbles will slide past another.
Very rarely
will a perfect raft of bubbles be formed; more usually, large bubbles or missing bubbles will
show as discontinuities — a model of dislocations in a three-dimensional crystal.
