It occurred to me as I was starting this blog that there are a lot of specialized terms in our field. My initial thought was to create blog entry giving definitions of all the terms that I could think of. But who, really, wants to read that? So I'm taking a different tack; I'll introduce the terms one at a time, with lots of pictures. Let's start with the most obvious one term.
What are liquid crystals?
Liquid crystals are ordered liquids. Ordinary liquids (water, gasoline, oil, etc.) are disordered at the molecular level. Liquid crystals, on the other hand, are made up of molecules that display some form of order. Molecular ordering is a characteristic usually associated with crystalline solids (e.g. sugar and salt). The main difference between liquid crystals and crystalline solids is their degree of ordering; the molecules of liquid crystal phases are not held in a perfect 3D matrix, as is the case for most solids. They have wiggle room. They can flow. That's the liquid part.
Ordered fluids have unique advantages, not the least of which is that they can be used to make LCDs (which stands for "liquid crystal display"). I will return to LCDs at some later date. One of their interesting features is that they give pretty textures when viewed by POM. In contrast, ordinary "isotropic" liquids appear completely black in polarized microscopy.
Ordered fluids have unique advantages, not the least of which is that they can be used to make LCDs (which stands for "liquid crystal display"). I will return to LCDs at some later date. One of their interesting features is that they give pretty textures when viewed by POM. In contrast, ordinary "isotropic" liquids appear completely black in polarized microscopy.
We can see this effect when we heat a liquid crystal; at some temperature, the molecular ordering of the material breaks down, and we are left with a plain old isotropic liquid. At this point, known as the clearing temperature, both the liquid crystal and isotropic liquids coexist. In the image of 4-(trans-4-pentylcyclohexyl)benzonitrile (right) the brightly colored regions are the liquid crystal phase, the black area in the upper right is the isotropic liquid phase. |
There are some caveats to this generalization that we'll explore more fully in subsequent posts. For example, the black wavy lines that snake their way though the LC phase are not actually isotropic regions. These "disclination lines" are a common feature of many liquid crystals, especially nematic phases. For example:
I'll leave you with one last image that nicely shows the fluid nature of the nematic phase; here, the rivulets of liquid crystal extend into the region of ordinary liquid: