Phase Transitions – Series II – 4


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.


Phase Transitions – Series II – 3


The third stage in the second series of Phase Transitions. The lattice of the crystals is changing and this puts them under mechanical stress. As a result, they adapt their shape by twinning (a zigzagging of the crystal lattice), resulting in striped patterns. I find this image visually very fascinating.

There is an intermediate stage (see below) but here something went wrong with the scanning of the slide. There is obviously some file corruption here in the lower stripe of the image. I will have this slid rescanned and will then repost the image. Maybe one bit was flipped. If a cosmic ray or a fault on the hard disk or whatever caused this, I don’t know. However, visually, it is an interesting picture, not least because of the mistake. In this picture, the stripes of the twinning are just beginning to appear.


 Pictures: Svend Keller

Phase Transitions – Series II – 2

DSC00288The second stage in the second series of Phase Transitions. Compared with the previous stage, the crystals are growing and some more of them are showing up.

The circular dark spots on the lower left are probably bubbles of air enclosed between the surface of the heating table (a heat resistant glass coated with a thin layer of metal that can be heated electrically) and the cover slip.

Why are such structures beautiful? Let me share some thoughts from a comment I made, answering a comment to a previous post in this series.

I think there is a lot of unseen (potential) beauty in the microworld. I think structures with some mix of order and disorder have a potential of being perceived as beautiful (I have written about this in the article On Beauty before).

If you look at such stuff macroscopically, it would just be a whitish crust on the glass. It looks uniform and structureless.

Now if you zoom in, you would start seeing something like a texture, until you reach this scale where you have just a few crystals in the image, with some structure (like dendritic branching). On this scale, you have structure with order (crystals) and disorder (different sizes and orientations). If you zoom in more, you end up seeing just one crystal filling the whole picture, or a small section of it, so you have a totally ordered, boring picture again. So to a large extent, the trick might be just to find the right scale. Zoom in an out and at some point, there is a maximum of beauty or a potential of beauty because there is a mix of order and disorder.

Phase Transitions 5


This is the last stage in this particular series of Phase Transitions (more are forthcoming).

The crystallites of the low temperature phase have taken over completely.

If you look at the whole series, you can see how these images are emerging from the interplay of physical laws and historical accident. Laws of nature determine the properties of these crystals, but accidents determine the exact direction and position of them. The initial melt is of a featureless symmetry, treating all places and all directions in the same way, at least on the scale visible through the microscope. When crystals form, this symmetry is broken, and domains form in which certain directions have different properties. In the images, this is reflected in the emergence of color and structure. Some of this structure is then obliterated in further phase transitions which create new symmetries and break existing ones, leading to a complex but partially ordered image that we perceive as beautiful.

The chance components of these processes are unpredictable, leading to a different outcome each time you perform this experiment, so this is an example of a system that generates new information. In a sense, this little blob of matter under the microscope is creative.

Phase Transitions 4

DSC00272The fourth phase of the Phase Transitions.

You may just look at this as a piece of abstract photography, and Svend Keller, when making these pictures, probably also looked at them from this perspective, with the attitude of a graphics artist and photographer.

On the other hand, as a material science teacher, he was also interested in the scientific side of it. If this aspect does not interest you, you might just stop reading here.

The crystallites of the low temperature phase are growing further, leaving only small rests of the previous configuration with the larger crystals striped by twinning (see previous posts). Some patterns of the previous dendritic growth pattern are still visible, perhaps as a result of different concentrations of the two components of this system, and the larger gaps are still visible. Apart from that, the crystals are “forgetting” their old structure. I don’t know if the gaps appearing black here were still filled with molten material or with another configuration (perhaps of only one of the two substances involved (Suberic acid and Phenyl salicylate (Salol))) that was not “optically active”, i.e. does not cause interferrence colours when interacting with polarized light.

Phase Transitions 3


The next phase transistion is taking place. In the previous one, whole cristals changed their configuration at once, in a process one may think of as a shearing of the whole molecular lattice. You may think of a group of soldiers turning on comand. The next phase transition, however, is a “civilian” transformation. Below a certain temperature, the old crystal lattice becomes instable. The transformation starts accidentally in some places, probably triggered by some faults in the crystal lattice. Then, one by one, molecules are shifting from the old to the new lattice, so the crystalites of the new conformation are growing bit by bit. For some time, the two phases coexist, but if the temperature goes down further, the new crystalites take over, obliterating most of the history of what was there before.

So from liquid to solid, from one solid phase to another solid phase and now to the third one, this is the fourth phase transition happening in this material.

Look at the previous phases of the Phase Transitions series to see more.