What is a Liquid Crystal?

Isotropic / Anisotropic Behaviour

Fluids (i.e. liquids and gases) are said to be ‘isotropic’, which means that all measurable properties of the material do not depend on the direction of measurement (X, Y or Z axis). In reality, because fluids have a random internal ‘structure’, all measurements are averaged, and this is what gives the property a uniform value. We all know that taking the average – i.e. the statistical mean – of any variable ignores its variance over time or distance, but let’s leave it at that, shall we? In the case of fluids, isotropy is a direct result of random (Brownian) motion since this is what results in the random ‘structure’ of a fluid. In contrast, some crystalline solids (as well as some amorphous solids such as wood) are ‘anisotropic’ because the concentration of atoms is different depending whether you measure it in the X-axis, Y-axis or the Z-axis. This results in the optical, thermal and electrical properties varying with direction. If we were to take a perfectly cubic structure, with all atoms positioned equidistantly (like in the ‘simple cubic’ structure we see below), then such a material would indeed be isotropic. However, most crystalline solids are not perfect cubic structures. Many are ‘body-centred cubic’ or ‘face-centred cubic’ where the anisotropy is visibly apparent. The only element that crystallises in a ‘simple cubic’ structure is polonium.

Birefringence in Liquid Crystals

If liquid crystals are viewed under a polarising microscope at low temperatures, they behave like a low-symmetry crystal and can manipulate polarised light, whereas at higher temperatures they do not. The anisotropic nature of liquid crystals results in what is called ‘birefringence’, where light that enters the crystal ‘splits up’ into two oppositely-polarised rays that travel at different velocities through the crystal.

Birefringence in liquid crystals

When viewed through crossed polarising filters, a birefringent material can exhibit beautiful patterns and colours. This is due to interference between the so-called ‘ordinary ray’ and the ‘extraordinary ray’; the latter takes a slightly longer path through the material and emerges at the other end out-of-phase with the ‘ordinary ray’. When the rays recombine, the interference produces colourful patterns. Furthermore, since liquid crystals expand with rises in temperature, this dimensional increase can delay the ‘extraordinary ray’ even more with respect to the ‘ordinary ray’, resulting in colourful patterns being visible only when the material experiences a rise in temperature. In effect, the liquid crystal acts as a temperature sensor! If liquid crystals are painted onto a paper strip and placed on a patient’s body, the liquid crystal strip can reveal infections or tumours, since these cause anomalies in local blood flow and hence in local body temperature.

The Pioneers of Liquid Crystals

Friedrich Reinitzer (1857-1927), Austrian botanist and chemist

Liquid crystals were first discovered in 1888 by Friedrich Reinitzer, who was studying cholesterol in plants, when he observed that the material ‘cholesteryl benzoate’ had two melting points. The lower melting point resulted in a ‘cloudy liquid state’ and the higher one in a ‘clear liquid state’.

Otto Lehmann (1855 -1922), German physicist

Reinitzer was perplexed by these observations and sent his findings to Otto Lehmann, who realised that these findings hinted at ‘a new phase of matter’. It was Lehmann who coined the phrase ‘liquid crystal’ in his article ‘On Flowing Crystals’ and is therefore attributed as the ‘father of liquid crystals’. His work did not however result in significant recognition.

Pierre-Gilles de Gennes (1932-2007), French physicist

That honour went to Pierre-Gilles de Gennes who developed a theoretical model to explain the properties of liquid crystals and their ability to scatter light, for which he was awarded the 1991 Nobel Prize in Physics.

Liquid Crystal Order

Liquid crystals contain rod or disc-shaped structures (called ‘mesogens’) which point along a common axis called a ‘director’. They are easily polarisable (typically with an electric field). Whereas solids are highly ordered, and liquids (and gases) have no long-range order, the molecules of a liquid crystal lie somewhere in between. At the first ‘melting point’ detected by Reinitzer (145ºC), the molecules lost some of the strict ordering typical of a crystalline solid, thus turning into liquid crystals. As the temperature rose to the second ‘melting point’ (178ºC) the liquid crystal then lost order entirely, becoming a transparent liquid.

Liquid Crystal Phases

Liquid crystals can be divided into ‘phases’ which describe their composition. These are termed Smectic, Nematic and Cholesteric phases:- In the Smectic Phase, the liquid crystal molecules form into layers, as can be seen in the above image. Individual layers can slip over each other, as observed in graphite. There is short-range order only within each layer. Nematic liquid crystals behave like ‘toothpicks in a box’, and maintain their orientation but are free to move in any direction. The rod structures form ‘electrical dipoles’ (they are electrically charged at each end, much like a magnet has a north and south pole) and can thus be controlled by an applied voltage. This has given rise to liquid crystal devices common since the 1970s, including LCD wrist-watches, calculators and flat panel TV screens. Cholesteric (or more correctly ‘chiral nematic’) liquid crystals comprise layers, each of one molecule thickness, with each layer arranged with their long axes parallel to each other. They exist as left- or right-handed twisted helices that can rotate the plane of polarised light. If the molecules are electrical dipoles, this twisting can be manipulated by an applied voltage, resulting in electro-optic shutters which switch the light on or off in the order of microseconds.

Liquid Crystal Dependence

The molecular forces that bind liquid crystals together are generally weak, and thus can be manipulated by mechanical stress, electromagnetic fields, temperature and by chemical composition. Consequently, we can identify three main kinds of liquid crystals:-

• Thermotropic liquid crystals: those that experience phase changes due to temperature
• Lyotropic liquid crystals: those that experience phase changes due to the density of a liquid
• Metallotropic liquid crystals: those that respond to low-temperature inorganic, metal-based compositions.

One surprising place where we find liquid crystals is in living bodies. Human bodies are 98% water, and this results in lyotropic liquid crystal phases developing on the internal cell walls. These phases allow the cells to remain flexible and to perform different tasks.

Outlook

Liquid crystals have revolutionised consumer devices since the 1970s and now find themselves in smartglass privacy windows and high-resolution displays for near-eye wearables as well as augmented reality headsets. This technology has the potential to impact society further as research continues into how liquid crystals can be applied in the transportation, industrial, medical and pharmaceutical sectors.

References

1. Introduction to Liquid Crystals, URL 2. “Single Crystal, Polycrystalline, Amorphous” – Introduction to Materials course (Texas Agricultural & Mechanical College), URL 3. Structures of Crystalline Solids, LibreTexts in Chemistry, URL 4. “Colour and the Optical Properties of Materials”, 2nd Ed. Richard Tilley, ISBN 978-047-074-6967 5. Liquid Crystals – Science Direct, URL