Materials Advent 2018 part 7 – Liquid Crystals

An image shows bright regions of color on a black background. The left shows features reminiscent of oil spills. The right shows multiple butterfly-like features of four same-colored spots around a central point.
A polarized light image shows the patterns caused by two different arrangements of liquid crystals within the same sample. From Wikipedia: Liquid crystal

Liquid crystals are everywhere now (if you’re looking at this on a computer, your display is probably LCD, and a decent chance any TV you looked at lately is an LCD too). They’re another one of those weird in-between states like we see in rheology. Liquid crystals are liquids based on their mechanical properties, but their molecules show some large-scale ordering that resembles solid crystals. This is because the molecules in liquid crystals tend to be relatively large, like a polymer chain, so you make distinct structures by lining them up, but it can also still be hard to pack them closely to make a true solid. The first liquid crystal was actually found studying cholesterol!

Rods pointing in the same direction but randomly spaced out.
A “nematic” liquid crystal – the molecules are lined up in the same direction, but there is not clear pattern in how they are spaced. 

Most LCDs are based on liquid crystals in the nematic phase seen above. (Although evidently not LCD TVs) Because the molecules are asymmetric, they can be oriented by electric fields because their electrons will be pulled by the electric force. The power in an LCD is basically to turn on and off the voltage to twist the crystals. This twisting is done to change how light passes through. A bunch of nematic liquid crystals lined up next to each other essentially act as a filter called a polarizer and line up the waves of light passing through. At the top and bottom layer of your display are two permanently oriented filters that are perpendicular to each other, an arrangement which would not let any light pass through without the crystals being lined up in a way to help align the light somewhere in between. (This is also why you can sort of see the pixels of LCDs at off-angles and why the picture can look so off away from the center of LCD TVs – the light is really only lined up for someone looking straight through the display.)

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