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.
PHOTO: California Red-legged Frog
by Wonder Science
Photo credit: CSIRO
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READ: Bubble Iridescence
by Peter Della-Rocca
The Molecular Structure That Causes Enthralling Light Effects in Soap Bubbles —
Soap bubbles, peacock feathers, and CDs all have one notable feature in common: all three are iridescent, they appear to change color when viewed from different angles. In the case of soap bubbles, this quality stems from the structure of a bubble’s surface. That shell consists of a layer of water pressed between two layers of soap molecules. Each one of those soap molecules has a hydrophilic end that is attracted to the water and a hydrophobic end that points away from it.
That double layer of soap molecules causes light that strikes the bubble to interfere with itself, such that the wavelengths of light striking the bubble are different from those that bounce back. Some of that light hits the outer layer of the bubble’s surface and some of it hits the inner layer. Because light can act like a wave, and because the light that hits the inner layer has traveled slightly farther, it is out of sync with the light that hits the outer layer, causing the two to interfere with each other, much like ripples in a pond where two stones have been dropped in near each other. Depending on how far apart the peaks of the two waves are, that interference might be constructive, amplifying the light as it bounces back, or it might be destructive, causing it to cancel itself out. The interference accentuates some of the colors that combine to constitute white light and it nullifies others, producing the shifting rainbow effect on the surface of a soap bubble.
The thickness of the bubble is not equal everywhere, and will vary over time as parts of it stretch or evaporate. A change in thickness alters the constructive or destructive interference of light waves, and causes different colors to dominate. The change in color that a bubble appears to have when viewed from different directions happens for similar reasons; the difference in distance traveled between light that strikes the inner layer and the outer layer will vary slightly when the light hits the bubble from another angle, and that change in distance yields subtle and beautiful variations in interference.
Like many aspects of soap bubbles, their iridescent quality is simple in principle but in practice yields enthralling effects.
Now you’re ready to float up up away on some pretty pink “Bubbles”.
About the Author:
Peter Della-Rocca is a policy nerd with a fascination in science and technology.