This is the last slide in the second series of Phase Transitions. Looking at these pictures, I find it amazing how such a beautiful structure can arise by itself. We see here a mixture of the laws of physics and the accidental history of the events under the microscope.
Two substances where mixed and melted, so from two white powders, we got a liquid drop. This was the first phase transition, from solid to liquid. The old order of the crystals in the powders was forgotten. The result is an unordered jumble of molecules moving around.
When they were cooling down again, crystals started growing. This was the second phase transition, from liquid to solid. Where these crystals grow and in what direction is a matter of accidents. It might be little bits of dust, but maybe just an accidental arrangement of some molecules to form the “seed” of a crystal. This little bit of order then reproduces itself over a large section of the melt, until it hits against another crystallite. Compared with the liquid, the overall order increases. Meanwhile, symmetry decreases: while the liquid does not favor any direction, so that it has spherical symmetry with infinitely many symmetry axes, in the crystal different directions have different properties and there are only a few symmetry axes or planes of symmetry or points of symmetry left (depending on the kind of crystal).
I a further stage, these crystals changed their configuration, from one ordered lattice to another one. This third phase transition leads from one solid to another, with a different lattice structure. In this particular case, the transition leads to a change in the shape, creating mechanical tensions. As a result, patterns of stripes arise, partially regular, partially irregular.
The beginning of a further phase transitions that is going to destroy this structure can be seen in a few spots.
The result of these steps is a complex mix of order and disorder. The interference colors, depending on the thickness and orientation of the crystals, resulting from the interaction of polarized light and the “optical activity” of the crystals, add to the beauty of the result.
The last transition, however, happens in the brain and mind of the observer. This step, I have to leave to you.