What is a Liquid Crystal?

Introduction

Fluids (liquids and gases) are ‘isotropic’, meaning that all measurable properties of the material do not depend on the direction of measurement.

In reality, because fluids have a random structure, we take the average of the measurements, and this is what gives the property a uniform value. As we know, taking the average of any variable ignores its variance over time or distance.

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 are ‘anisotropic’ because the concentration of atoms differs depending whether you measure it in one direction, or another. This is also true of some amorphous solids like wood.

This results in a variation of its optical, thermal and electrical properties depending on direction.

If we consider a perfectly cubic structure, with all atoms positioned equidistantly (like in the diagram below), then we consider this material to 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. In contrast,  at higher temperatures they cannot.

The anisotropic nature of these materials results in ‘birefringence’, where light that enters the crystal splits 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 ‘ordinary ray’ and the ‘extraordinary ray’; the latter taking a slightly longer path through the material and emerging 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 delays the extraordinary ray with respect to the ‘ordinary ray’, resulting in colourful patterns, which are visible when the material warms up.

Essentially, the material acts as a temperature sensor!

If liquid crystals are painted onto a paper strip and placed on a patient’s body, we can detect infections or tumours, since these change local blood flow and hence local body temperature.

Pioneers of Liquid Crystals

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

Liquid crystals were first discovered in 1888 by Friedrich Reinitzer, while studying cholesterol in plants.

Reinitzer observed that 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 contacted 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 this technology. 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 won 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 polarisable when we apply an electric field.

Whereas solids are highly ordered, and liquids (and gases) have no long-range order, liquid crystals lie somewhere in between.

At the first melting point (approx. 145ºC), the molecules start to lose the strict ordering found in crystalline solids, and start turning into liquid crystals. As the temperature rises to the second melting point (approx. 178ºC), the material loses 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:-

Smectic Phase

In the Smectic Phase, the 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 Phase

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), where an applied voltage can change their orientation.

This gives rise to LCD wrist-watches, calculators and flat panel TV screens.

Cholesteric (chiral nematic)

Cholesteric 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: those that experience phase changes due to temperature
• Lyotropic: those that experience phase changes due to the density of a liquid
• Metallotropic: 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 they 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