An introduction to liquid crystals

If you’re a regular reader of this blog you’ll know that there’s more to the world around us than the three states of matter that we learn about at school. In the short time that we’ve been writing we’ve looked at plasma, dark matter and colloidal systems, all of which challenge the traditional view that everything around us can be classified as a solid, a liquid or a gas. In this post, we’ll continue this theme by introducing the bizarre world of liquid crystals.

You’re probably more familiar with liquid crystals than you realise. In fact, there’s a high chance that you’re reading this blog post on a device that contains an LCD – a Liquid Crystal Display (unless you’ve got one of those fancy OLED displays that are around nowadays, or are still rocking a plasma!).

Liquid crystal displays are everywhere nowadays, with most people owning several¹, but not many people know what the stuff inside these displays actually is. In this two part series (read that in a dramatic TV announcer style voice!), we’ll try to rectify this situation. This post introduces liquid crystals and sets up part 2, where we’ll look at why they’re used in displays.

What’s in a name?

It’s a strange name, right? The term ‘liquid crystal’ seems like an oxymoron – in our traditional solid, liquid, gas view of the world a ‘crystal’, which is solid, and a ‘liquid’ are two distinctly different things. Clearly a ‘liquid crystal’ combines these two things in some way, but how? Given their name, you’d be forgiven for thinking that that liquid crystals are bits of crystals that float in a liquid or perhaps that they’re crystals that flow like fine sand does, but in fact they’re something much weirder!

A quick google search will reveal that liquid crystals are “an intermediate state of matter between a solid and a liquid”. We’ve seen a kind of intermediate behaviour before when we examined gels. So are liquid crystals like gels? Well, no, they’re something quite different… as we saw in our previous post gels are colloids (one state of matter dispersed in another); however, liquid crystals are a state of matter in their own right that display some of the properties of solids, and some of the properties of liquids.

To better understand how this can be the case, it’s useful to examine how liquid crystals are formed. In this regard, we can define two general types of liquid-crystalline materials:

• Thermotropic liquid crystals, which are materials that exhibit liquid-crystalline behaviour as a function of temperature.
• Lyotropic liquid crystals, which are materials that exhibit liquid-crystalline as a function of concentration when mixed with a suitable solvent. Lyotropic liquid crystals are closely related to micelles, which we examined here.

In this post, we’ll focus on thermotropic liquid crystals, which are the types of materials used in LCDs. We’ll save lyotropic liquid crystals for a separate post (maybe Part 3 of this series?!).

Thermotropic liquid crystals

Most of the materials that we encounter in everyday life can exist as a solid, liquid or a gas. For good old H₂O, for example:

• Below 0 °C water exists as ice, which is a solid
• Ice melts at 0 °C to give liquid water
• Liquid water boils at 100 °C to give gaseous water

So, when heating water from -10 °C to 110 °C, we encounter two phase transitions:

• The melting point, at which solid water (ice) turns into liquid water
• The boiling point, at which liquid water turns into gaseous water

Nothing new here – the thermal behavior of water is something that we’re all familiar with and many of the other materials that we encounter in our everyday lives behave in a similar way, with the only difference being the temperatures at which they melt or boil.

Thermotropic liquid crystals, on the other hand, are different in that they have at least one additional phase transition. If we take a thermotropic liquid crystal in its solid phase and heat it, at some defined temperature (the melting point) it will melt, not into a liquid but into a liquid-crystalline state. If we then continue heating, one of two things will happen:

1. The material will undergo a phase transition and a liquid will be formed; we call the liquid crystal to liquid phase transition the ‘clearing point’, or
2. The material will undergo one, or more, transitions to other liquid crystal phases as the temperature increases before we eventually reach the clearing point temperature and a liquid is formed

If we keep heating the resulting liquid, we’ll eventually reach the boiling point and a gas will form (assuming that the material doesn’t undergo thermal decomposition before we get there).

So, thermotropic liquid crystals have one or more liquid crystal phases sandwiched between the conventional solid and the liquid phases. For this reason, you’ll sometimes hear liquid crystal phases referred to as ‘mesophases’ – the prefix ‘meso’ indicting these phases are intermediate (between the solid and liquid phases).

At this point, there are a couple of interesting points to note:

• Firstly, the mesophases formed by thermotropic liquid crystals are stable – if a material is in the liquid-crystalline state, it will remain in the state forever, providing that the temperature doesn’t fall below the melting point or exceed the clearing point. In other words, the liquid-crystalline state behaves like the solid, liquid and gaseous states – materials that we know to be solid at room temperature don’t melt unless we heat them and materials that we know to be liquid are room temperature don’t freeze unless we cool them!
• Secondly, the liquid-crystalline state exhibits some of the properties of the solid phase and some of the properties of the liquid phase – this is what makes liquid crystals so useful in displays, as we’ll see later.

So, the liquid-crystalline state is an intermediate state of matter between a solid and a liquid – but what what kinds of materials form this kind of phase, and what do mesophases look like?

8CB – an example of a thermotropic liquid crystal

At this point, you’d be forgiven for thinking that there must be a very limited number of real-life materials that exhibit the weird behaviour that we’ve described above, but you’d be wrong. There are a lot, and I mean a lot, of materials that are known to form mesophases. We’re talking tens of thousands, probably more… but of all of these known materials, one family is particularly well known: the cyanobiphenyls.

Remember your old calculator with the dodgy black and white LCD display? There’s a good chance that it contained a mixture of cyanobiphenyl-based materials. These materials are, in part, what enabled the LCD revolution and are still widely used in liquid crystal research today. Cyanobiphenyl liquid crystals are based (perhaps unsurprisingly) on a cyanobiphenyl core, to which is attached a hydrocarbon chain. Here, we’ll look at just one of these materials, which has 8 carbon atoms attached to the core (8CB – where CB stands for ‘cyanobiphenyl’), as shown in the diagram below. For those of you that are not familiar with chemical structures the zigzag line represents the 8 carbon atoms and the two hexagons with ‘CN’ represent the cycanobiphenyl bit.

This material melts, at 22 °C to form a mesophase which we’ll call M1, exhibits a phase transition to another mesophase, which we’ll call M2, at 34 °C and has a clearing point of 42 °C, above which it exists as a conventional liquid. Note here that we’ve used double headed arrows to indicate phase transitions exhibited by 8CB – this is because the phase transitions exhibited by thermotropic liquid crystals are thermally reversible, i.e. the phase transition between M1 and M2 occurs at the same temperature on heating or cooling.

The phase behaviour of 8CB raises interesting questions:

1. What is it about 8CB that makes it a thermotropic liquid crystal?
2. What is the structure of the mesophases, M1 and M2, that it forms?

The answer to question 1 is quite complicated, but one major factor is the molecular shape of 8CB. Notice from the diagram above that 8CB is quite a long molecule – it is longer than it is wide. A fancy way of saying this is that it has an anisotropic molecular shape. Materials that form thermotropic liquid crystals are almost always composed molecules that are either long and thin, like a rod, or short and fat, like a disc (other shapes work too, but rods and discs are most common). Other factors, like intermolecular interactions, also play an important role, but molecular shape is key.

To answer question 2, let’s return to familiar ground and think about solids and liquids for a minute. Generally speaking, we can think of a solid as a three-dimensional arrangement of molecules. In contrast, the molecules in a liquid are more mobile and are able to move/rotate relatively freely. The transition between a solid and a liquid occurs at the melting point; it is characterised by a breakdown of the three-dimensional solid network and an increase in the mobility of the molecules within the material that we’ve just melted.

So what’s happening when 8CB melts? Well, unlike normal materials, in which all of the order in the three-dimensional solid network is destroyed at the melting point, only some of the order that was present in the solid phase is lost when 8CB melts. If we look at simplified diagram of the crystal structure of 8CB, shown below, we define two types of order:

• Positional order – the molecules are arranged layers and are ordered within these layers
• Orientational order – the long axes of the molecules are aligned with one another

When 8CB melts at 22 °C to form the first mesophase, not all of the order found in the solid state is lost. The mesophase formed still has layers, but the layers are much less well defined than in the solid state, and the molecules are less well ordered within the layers, as shown below.

In this arrangement, there is still a degree of positional order because the molecules are loosely arranged in layers, and some orientational order as the long axis of the molecules are roughly aligned. However, there is also a degree of fluidity – molecules rotate more freely than in the solid state and are able to move through and within the layers. We call this type of arrangement a ‘smectic’ mesophase². There are many different types of smectic mesophases.

Now let’s imagine that we heat up our smectic mesophase. In doing so, we will encounter a phase transition at 34 °C at which the layered structure breaks down but some orientational order remains, as shown in the diagram below.

This arrangement has no positional order, but does have orientational order because the long axes of the molecules are still roughly aligned aligned. We call this phase the ‘nematic’ phase³ (not to be confused with Pneumatic – there are no drills involved here!).  This nematic phase is this phase that is used in Liquid Crystal Displays. We’ll see how the nematic phase formed by materials like 8CB can be used in displays in part 2 of this series.

If we were to keep heating our sample of 8CB, now in the nematic phase, at 42 °C we’d reach the clearing point. This is the temperature at which the orientational order breaks down and a conventional liquid is formed.

It should be noted that other liquid-crystalline materials transition directly from the smectic phase to a liquid or directly from the solid phase to the nematic phase. In fact, there are many liquid crystal phases and different materials show different phase sequences; the exact phase sequence observed in a given thermotropic liquid crystal is dependent on many factors, including molecular shape and intermolecular interactions. Since the aim of this post is just to give you an introduction to liquid crystals, we won’t go into more any more detail here on this here, but this Wikipedia page has some good information and suggestions for further reading, if you’re interested.

End of Part 1

Although we’ve covered quite a lot in this post, we’ve only just scratched the surface of the liquid crystal world. As we mentioned above, there are many, many materials that form mesophases and many different types of mesophases that can be formed – far too many to cover in this blog post! Hopefully this post has, however, given you a general idea of what liquid crystals actually are and maybe helped to reinforce that the world around us isn’t just solids, liquids and gases!

It’s worth noting that the images in this post really don’t capture the dynamic, fluid nature of liquid crystals; we’ve used static pictures above, but in reality the molecules in the nematic phases are constantly moving and rotating rapidly around their long axes⁴. If you can spare a few minutes, it’s well worth watching a few videos to get a feel for how they look in real life, and to see the amazing patterns that they form when examined using polarised optical microscopy – you can find a good YouTube playlist here.

So that’s the end of Part 1. Keep an eye out on our Twitter feed @SciByDegrees for part 2, where we’ll look at some other interesting properties of liquid crystals and see why these materials are so useful in displays…

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¹Think about your phone, TV, car dashboard display, calculator, iPad, digital watch etc…

² The word ‘smectic’ comes from the Greek word ‘smectos‘, which means soap. The structure of the smectic phase is very closely related to the structures adopted by the molecules in soaps.

³ The word nematic comes from the Greek word ‘nematos‘, which means thread, or thread-like. When observed by polarised optical microscopy (a common technique used to examine liquid crystals) the nematic phase appears to contain thread-like structures (see here, for example).

⁴ The molecules also rotate around their short axes, but at a considerably slower rate – if they were rotating freely about this axis (as they do in the liquid state) there would be no orientational order, i.e. the long axes of the molecules wouldn’t be aligned.