These two pictures show crystals of a mix of two organic chemicals under a microscope. These pictures where made by my father, Svend Keller. I am posting them here for their aesthetic qualities. Further down in the article, I am giving a description of how they where made and what exactly it is, for the science minded among you. The second picture is the same crystals as in the first, after cooling down and undergoing some changes in their internal structure as a result.
I am planning to post more such pictures because I find them very beautiful. However, the other ones I have are slides and I have to have them digitized first.
The colors are a result of passing the light through polarizing filters. Without that, these crystals are colorles.
So what exactly is this? If you are not interested in the scientific aspects of this, you can skip the following and just enjoy the pictures, but maybe some of you would like to know.
With respect to the colors, the science minded among you may search the web and the scientific literature for how this creates colors (look for “polarization colors”), I never bothered myself to understand the exact physics of polarization colors so I cannot explain them. Looking through a microscope at such structures, I just know they are beautiful.
The pictures where made with a “heating table”. That is a small rectangular metal frame (about 10 centimeters wide) with a glass pane on top (so it looks like a small table, hence the name). The glass is a special glass that does not shrink or expand much when heated or cooled. Think of heat resistant glass dishes. The glass is coated with a very thin layer of a metal (I think it is gold, but I am not sure). This layer is so thin that it is transparent (think of the metal-coated windows of some office buildings). This metal layer is connected to an electric transformer with a turn switch. Since the metal layer is very thin, it has a high electric resistance. Let electricity flow through it and it becomes hot as a result. So it is like a small electric cooker.
The heating table is put under the microscope and then you put small amounts of chemicals under it (in powder form). These must be substances that can melt when heated, without being destroyed by the heat (so don’t try it with things like sugar).
In this case, my father used two substances, Suberic acid and Phenyl salicylate (Salol). He put on the electricity, so the heating table became hot. After a few seconds, the two substances where melting and mixing.
Now he lowered the heat (I don’t know if he turned the current down or switched it off). As a result, crystals started growing. The result can be seen in the top picture. The grey areas are rests of molten material.
Unfortunately, I don’t have a film of this. It is actually exciting to watch the crystals grow before your eyes.
The crystals show a branching pattern known as “dendritic growth”. This means that the crystals develop side branches, giving a feather-like structure. In some experiments, you can see these side-branches developing further side branches again. The result is a fractal pattern that shows similar structures on larger and smaller scales. The reason for this is that the same laws are acting on larger and smaller scales, so that similar processes will happen in the large and in the small. Aesthetically. this creates some order across the scales, resulting in strong aesthetic effects.
An example of such a fractal pattern can be seen here: The large crystal that has grown from the middle to the upper right (blue) developed a number of side branches, (to the right side). On the third level, however, further branching was not possible since the branches where so dense that there was not enough space and molten material was left between them, resulting in the feather-like shape you can see.
When the material cooled down further, the crystals underwent a change of the crystal configuration. A crystal is a material in which the molecules are arranged in an ordered repeating structure called a lattice. Some materials have several different possible configurations. At certain temperatures, a change from one structure to another will occur. This is a “phase change” just like melting or solidifying, but instead of changing from the liquid phase to the solid phase, the material changes from one solid phase to another. This changes the optical properties of the material, resulting in a change of color. It also results in a change of shape. Since the crystals are surrounded by other crystals, changing their shape will put them under tensions. In this particular mix of materials, the resulting forces cause the crystal to adapt its shape through a process called twinning. In twinning, the lattice will shear into a different direction, resulting in stripes of different orientation of the crystal lattice. These are visible in the second picture as fine lines across the original crystals. Sometimes, these stripes are then themselves divided up crosswise in a second round of twinning, again a process resulting i a fractal pattern.
Material scientists know a type of steel in which the same is happening. When cooling, a configuration change with a form adjustment through crystal twinning is happening in this type of steel. This is the particularly hard material known to experts as “Martensite”. In fact, on can say that the mix of suberic acid and phenyl salicylate is also forming Martensite. It is a phenomenon not limited to steel. I think my father discovered this organic Martensite himself when experimenting with different mixtures of different materials.
My father was a professional teacher teaching material science to people learning metal related professions. He used this as a model system to explain the formation of Martensite to his students. When you look at it through a microscope, you can actually watch it happening and understand the concept better. In steel, this is much harder to observe since it is happening at much higher temperatures. It would not be possible with the equipment of a school. The whole experiment just takes a few minutes and always results in very beautiful images.
Different substances, and especially mixtures, result in different types of shapes. The results are always interesting and beautiful. I have spent some time doing such experiments myself. In one instance I found a mixture of two substances (as far as I remember, that was camphene mixed with resorcin, but I am not completely sure, it is decades ago) in which the direction of crystal growth depended on the temperature. By regulating the electric current and hence the temperature up and down, I could create a wave-like colored pattern as a result. Unfortunately, I did not have a camera on that microscope.