2012: Advent Chemistry: Liquid Crystals

by on December 20, 2012

The structure of 4-cyano-4'-pentylbiphenyl

This is 4-cyano-4′-pentylbiphenyl, and it’s a liquid crystal.

Now, to cover liquid crystals properly would take up another Advent’s worth of posts, so I’m not going to. I’m going to simplify things horribly. I’m going to tell you lies-to-readers, in the same way that teachers tell lies-to-children. I know it’s not precisely correct, and you know it’s not precisely correct, but it will serve to get the basic concepts across.

Liquid crystals are materials, like that one up there, which exhibit a very particular set of properties. The first and most important one is this: they have non-isotropic liquid phases.

Normal liquids are isotropic, meaning they’re the same in all directions, because there’s no order or structure to them. Think of a crowded party with people milling about. Liquid crystals, by contrast, are not the same in all directions, because the molecules tend to organise themselves in some way. The most common way is the nematic phase. When someone stands up on a chair to make a speech, everyone in the room looks towards the person on the chair, and the party enters the nematic phase. The people are still randomly arranged, but they point in roughly the same direction.

Liquid crystals only behave this way between certain temperatures. Too cold, and they freeze, like pretty much everything. Too hot, and they reach a ‘second melting point’ and become ordinary isotropic liquids. ‘Melting’ twice is what tipped people off that there was something funny about certain molecules.

This behaviour, forming partially ordered liquids, would just be a curiosity if it didn’t come with some more interesting additions. Indeed, it was just a curiosity, until several things were worked out.

1: Liquid crystals of certain kinds are polarised, only letting light through if the light has the right degree of rotation. (You’ll have to take my word for it that light has degrees of rotation; we don’t have room for the explanation, and anyway, that’s physics.)

2: By hooking liquid crystals up to electrodes in various clever ways, it is possible to change the direction they point in and thus which direction they are polarised in.

By combination of points 1 and 2, we reach the following brilliant application: if you set things up right, you can pass a polarised light through a layer of liquid crystal and, by changing where the voltage is applied, change which parts of the layer of liquid crystal let that light through.

Polarising light is easy – just put it through a filter, a piece of polarised film that only lets a certain direction of light through. Put liquid crystal, clever electronics, and a polarising filter or two together, and hey presto – you’ve made a Liquid Crystal Display. LCDs are in everything from calculators to computers to alarm clocks to digital cameras, and they’re jolly clever.

So, finally, 4-cyano-4′-pentyldiphenyl. It’s a classic liquid crystal. The vital property is that it is long, narrow and reasonably rigid, so it’s naturally inclined to line up in long rows rather than tie itself in knots with its neighbours. Lining up produces order, and order produces LCDs.

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