A Mysterious State of Matter Powers Our Digital Displays
The three states of matter to which every foundational chemistry class introduces us — solid, liquid, and gas — turn out not to be quite so clearly delineated as they might seem. Liquid crystals represent a sort of mixed state, not quite solid but not quite liquid. While molecules that make up a solid are arranged in a fixed pattern and molecules in a liquid move freely, molecules in a liquid crystal will tend to point in a particular direction, but not with the uniformity of a solid.
A liquid crystal is said to have a director, an orientation that will describe some, but not all of the crystal’s molecules, as well as an order parameter, which indicates how much the molecules hew to their director and how much they move about at random. As the temperature of the crystal increases, the order parameter will tend to fall until the crystal transitions entirely to a liquid.
Depending on the pattern of movement that the molecules follow, a liquid crystal may take several different forms. Nematic, or thread-like, crystals are those made up of molecules that tend share the same orientation but may move freely as in a liquid. Smectic, or soap-like, crystals on the other hand, comprise molecules arranged in sheets that may slide over one another.
Examples of liquid crystals abound even in everyday places. Soap mixed with water can act as a liquid crystal, as the soap molecules’ hydrophilic ends attach to water molecules and their hydrocarbon tails cluster together. Additionally, the fatty myelin that shields nerve cells and, under the right conditions, human blood can also act like liquid crystals.
The behavior of liquid crystals lends itself to a range of applications, from thermometers to lasers, and, perhaps most commonly, liquid crystal displays, or LCDs. Liquid crystals were discovered in 1888 by Friedrich Reinitzer, an Austrian botanist who observed unique properties in Cholesteryl benzoate, but the technology that enables LCDs came in large part from Marcel Vogel, an often eccentric researcher who worked both with IBM and an independent company that he himself founded. Wonder Science is custodian of hundreds of different liquid crystals that Marcel Vogel synthesized in the 1960s and 1970s. You can marvel at the dynamic phase transitions of Cholesteryl benzoate and more in our Liquid Crystals video, with music by Ariel Pink.
The usefulness of liquid crystals in LCDs stems from the way the crystals interact with light. Light passing through such a crystal will behave differently based on the crystal’s orientation, meaning that it may appear either transparent or opaque. Further, because a liquid crystal does not behave wholly like a solid or like a liquid, its orientation can be manipulated by weak electric and magnetic fields. While the molecules that make up a solid are held too rigidly in place and those that make up a liquid move too randomly, a liquid crystal strikes just the right balance, such that it will rearrange itself in predictable ways. As a result, applying an electric field to a liquid crystal can alter the way it reacts to light, meaning that individual structures can be switched “on” or “off,” rendered transparent or opaque as their orientation changes. Flowing like a liquid and maintaining properties of solid crystals, liquid crystals bring us the best of both worlds. After all, liquid crystals are how you actually see “Game of Thrones” on your television set.