Science

Liquid crystals

Cold figures from coloured swirls -At the heart of a liquid crystal display lies an apparently chaotic liquid swirl that arranges itself in a precise order at the touch of a microcurrent.

The paradoxical problem-solvers

They're neither solid nor liquid nor gas - and they don't look like crystals, either. Yet these materials are crucial to our bodies as well as our electronics. And we're only just beginning to make use of them

We're all familiar with liquid crystals - those black figures silently changing in watches, or displaying text on a laptop computer. They have become a hallmark of our digital age.
As their paradoxical name implies, liquid crystals are Alice - in - Wonderland materials: they flow in the same way as liquids, yet they affect light like solids. This bizarre combination of properties is being put to use in some revolutionary technologies: they are likely to be the key to the new wall-sized television screens as well as the next generation of CDs, which will allow recording as well as playback.

Liquid crystals in action

Laptop display - Liquid crystal computer screens are already close to the quality of ordinary cathode-ray tubes-in a fraction of the space and without wasting power creating unwanted heat. Crystal-clear TVs -  Improvements to the switching of liquid crystal displays are enabling pocket television sets to become increasingly commonplace. Glowing thermometer  - Temperature- sensitive liquid crystal film shows a hand's hot spots - one of scores of medical and scientific applications

But liquid crystals are now beginning to turn up in some very surprising places too. Scientists think that liquid crystals may hold the key to certain medical disorders - including cancer. And most recently liquid crystals have turned up in a still more unlikely area: cosmology - the study of the origin and nature of the universe. Researchers in America think that liquid crystals can help them understand the events that took place billions of years ago, just after the Big Bang, when galaxies were being formed.

Liquid crystals are continually springing surprises on scientists even now, over a century after their discovery. Indeed, their behaviour is so odd that they were almost not discovered at all. Historians now know that some scientists actually saw natural liquid crystals under their microscopes as long ago as the 1850s - but simply wrote them off as curiosities.

These early sightings were made during experiments on a white fatty material known as myelin, which covers the nerve fibres in our bodies like the plastic insulation around the filaments in electrical cables.

How could these liquids have two quite different melting temperatures?

A number of scientists noted that myelin turned liquid when left in water. This liquid was distinctly odd, however: under certain lighting conditions, it produced spectacular colours - just like light being shone through a glittering crystal.

Myelin: the liquid crystal that guards your nerves Liquid crystals were observed as long ago as the 1850s, during experiments on human nerve fibres, where they were found in myelin - the nerve system's natural insulation system.

Clearly something strange was going on, but no one probed further. Then another curiosity emerged: these liquids seemed to have two different melting points. Not until the 1880s did the answer become apparent: instead of changing straight into a liquid when heated, these solid materials transform into a kind of intermediate state that emerges at the first melting point, and disappears at the second. Between these two temperatures, the material flows like a liquid yet keeps some of the optical properties of a solid crystal. In short, it has become a "liquid crystal".

Once their existence was confirmed, liquid crystals began to attract many other scientists around the turn of the century, keen to solve the mystery of their curious properties. German chemists - then widely regarded as the best in the world - made the first artificial liquid crystals, and theoretical physicists set to work on providing theories to explain their behaviour.

Their strange structure has been described as 'a fourth state of matter'. If our bodies' own essential liquid crystals fail we sicken and die. The meltdown of a liquid crystal has unlikely parallels with the early moments of the universe. LCD screens will form the basis of the entertainment of the future.

By the 1920s, it became clear what was happening. Below a certain temperature, the molecules of liquid crystals are neatly arranged, stacked like cans in a supermarket display. But heat up a liquid crystal and it starts to show its peculiarities. In a normal liquid, molecules are randomly arranged; but the molecules of a warmed liquid crystal retain some of their original orderliness - just enough order for the liquid crystal to retain the optical properties of a solid.

Heat a liquid crystal still further, however, and its molecules will eventually lose their order entirely. Its "solid" properties vanish, and it behaves as a conventional liquid should.
Scientists discovered that these changes can happen in a number of ways, giving rise to different types of liquid crystals. In a so-called smectic type, for example, slight heating jumbles the arrangement of molecules, but the solid retains a basic, layered structure. In nematic types, by contrast, the jumbling affects both the individual molecules, and the layers they originally sat in.

As early as the 1930s, some scientists were talking about putting liquid crystals to use in a new form of data display, where their optical properties - and thus what was on the display - would be controlled by temperature changes. Frustratingly, however, the early liquid crystals were made unstable by small temperature changes, and their behaviour was too erratic for practical use.

A spectacular solution for cataract sufferers

People with cataracts - a film that clouds eye lenses - see more clearly through a small slit: less light in the eye means less is scattered by the cloudy cataract. Now liquid crystal glasses have been developed that exploit the effect - but nevertheless allow cataract sufferers a wide field of vision. The glasses make use of the fact that liquid crystals turn from opaque to clear on application of a weak electric current. They have an array of about 30 tiny liquid crystal panels. When a current is applied to each panel in turn, a narrow horizontal slit appears in the lenses. The glasses compensate for a restricted view by moving the slits up and down at a speed too fast for the eye to detect The slit reduces the amount of light entering the cataract sufferer's eye; but it also moves rapidly up and down. It scans the visual field 50 times per second, allowing the wearer a bigger window on the world, without sacrificing clarity of vision. The glasses were invented by French physicist Siegfried Klein when he had cataracts himself. He emphasises that they do not replace conventional surgery. "But they may delay the need for an operation by five or six years," he says. "The glasses should also be useful for those sufferers who cannot have surgery because of their age or other eye disease." This is the first of many possible uses for liquid crystals in "spectacle" form. And of course there are many non-medical applications in the pipeline, including VR and stereoscopy.

The breakthrough finally came 40 years later, in the chemistry laboratories of the University of Hull. Building on the understanding gleaned over decades, George Gray and his colleagues succeeded in making the first really stable liquid crystals. Known rather forbiddingly as nematic alkylcyanobiphenyls, they were cheap as well as stable - and their optical properties could be precisely controlled using tiny electrical charges instead of heat.

Without their liquid crystal structures, living cells couldn't exist

This breakthrough marked the beginning of what has become the multi-billion-pound international liquid crystal display (LCD) industry. First to appear were the tiny voltage-controlled LCDs in watches and calculators, which simply showed figures and letters in black against a paler background. Unlike the light-emitting diodes they replaced, they used minute quantities of electric current. By the late 1980s, however, tiny colour televisions started to appear, based on differently oriented liquid crystals. Many computer displays now use them, and it is only a matter of time before really large colour LCD "flat televisions" reach our homes - far larger than could be made using conventional cathode-ray tubes, and far less power-hungry.

But these technological applications have masked the much wider importance of liquid crystals. Some of these hark back to the very early days, when it was the biologists rather than the physicists who were trying to understand their properties.
Every living organism on earth is made up of cells - squidgy bags of fat so small that 10,000 of them would barely cover the head of a pin. Life itself is sustained by these cells interacting with each other - taking in nutrient, putting out proteins. Yet, paradoxically, these same delicate cells make up the bodies of even the largest, toughest creatures. There is one type of material perfectly suited to both that can provide the strength and flexibility required by a living cell: liquid crystal.

Scientists now know the walls of living cells consist of liquid crystal materials called phospholipids. A dramatic illustration of this is provided by bacteria that grow deep in the ocean under tonnes of pressure per square metre. The phospholipids at this pressure retain enough liquid crystal structure at the ambient temperature to provide a strong cell membrane capable of withstanding the strain.But when the bacterium is brought to the surface, the pressure drop changes the critical temperature at which the liquid crystal cell walls can maintain their structure. They become completely liquid: the bacterium disintegrates, and dies.

Scientists have discovered that living organisms have learnt to control the liquid crystal properties of their cells precisely, so that the critical pure liquid temperature is always above the temperature of their environment - but only just. For the cell walls must not only be strong enough to protect the contents, they must also stay permeable enough for the processes of life to continue.  

Changing states: how a digital display works
	A simple black - or - white LCD display works by either allowing daylight to be reflected back out at the viewer or preventing it from doing so - in which case the viewer sees a black area. The liquid crystal is the part of the system that either prevents light from passing through it or not. The crystal is placed between two polarising filters that are at right angles to each other and together block light. When there is no electric current applied to the crystal, it twists light by 90o, which allows the light to pass through the second polariser and be reflected back. But when the voltage is applied, the crystal molecules align themselves, and light cannot pass through the polariser: the segment turns black. Selective application of voltage to electrode segments creates the digits we see.
If you heat a liquid crystal... A cool liquid crystal is solid (1) . A temperature rise melts it into a unique semi-liquid state (2) . Further heating makes it fully liquid  (3) .

Humans rely on the properties of natural liquid crystals for their well-being too. The breakdown of myelin, the substance wrapped around nerves, can have serious effects on the nervous system - including multiple sclerosis.
Although the precise cause of the breakdown is still mysterious, it is thought to be tied to the liquid crystal properties of myelin, on which its insulating qualities depend. A failure of the liquid crystal structure of the myelin causes an insulation failure, which devastates the transmission of nerve impulses. Sickle-cell anaemia - the blood disorder in which the red blood cells take on a peculiar shape and fail to perform adequately - is another example of the importance of liquid crystals. Because of a genetic defect, sufferers of this disease carry in their blood cells abnormal haemoglobin molecules with unwanted liquid crystal properties. These molecules give the blood cells a stickiness that dramatically reduces their efficiency as oxygen carriers, and leaves sufferers weak and feverish.

It is possible that natural liquid crystals could even play a role in cancer. Cancer - essentially the uncontrolled growth of cells - is notorious for starting in, say, the liver and spreading ("metastasising") to other organs.
Ordinary, healthy cells have arrangements of so-called microtubules around them. Like the regular pattern of molecules in a liquid crystal, this seems to provide a relatively rigid structure that appears to control the movement of cells. In cancerous cells, however, these microtubules are less ordered - and are thus less well able to stop the cells spreading to other parts of the body. Understanding why the microtubules have these liquid crystal properties, and why they lose them, will surely prove useful in the war against cancer.

Recently scientists have come up with perhaps the most surprising role yet for liquid crystals: as models for events that took place at the beginning of the universe.

As the liquid crystals cooled, the patterns were just like those in the early universe

Researchers trying to understand what happened billions of years ago, just after the Big Bang, face an apparently insuperable problem: they cannot go back in time to test their ideas in the original conditions. But this year, American scientists showed that liquid crystals may help cosmologists get around this problem.

Re-running the Big Bang in liquid crystal simulation According to current theories of the birth of the universe, the early cosmos underwent so-called "phase changes" as it expanded and cooled. But is the theory right or not? To find out, American researchers warmed up nematic liquid crystals, in a tiny simulation of the early universe. They then watched what appeared as the liquid crystal cooled. What they saw (shown right) agreed perfectly with the theory - and could explain where galaxies come from. The defects that formed could, in the real universe, have seeded the growth of larger structures - like galaxies.

(1) Bubbles appear after heating of the "mini-universe". (2) The bubbles start to merge, forming bigger structures. (3) Regions between bubbles become sharply defined. (4) These regions join up as "string defects". (5) Counting the remaining defects gives a test of cosmic theory.

According to current theories, the very early universe was incredibly hot and dense, and in an utterly different state from today. But as it expanded and cooled, the universe started to undergo radical changes: so-called "phase changes", rather like water turning into solid ice.
These changes did not occur uniformly, however: different parts of the universe "froze" at different rates and into different states, like the patchwork of frost on an icy windowpane. Where these contrasting regions meet, so-called defects form.

Cosmologists have tried to calculate how many such defects are generated around each region of the universe. They are keen to find out, because too many defects could lead to a universe quite unlike the one we know today - suggesting that current theories of the early universe could be completely mistaken.

Dr Mark Bowick and his colleagues at the universities of Syracuse and Santa Barbara realised that as they cool down, liquid crystals undergo very similar "phase changes" to the universe itself. So could liquid crystals point the way to our cosmic past?
To find out, the team heated up a nematic crystal, and watched what happened as it cooled. Under their microscope, they saw little regions of liquid crystal form and start to merge, just like parts of the early universe cooling down and "freezing" into various states. After about ten seconds, defects between the liquid crystal regions were clearly visible too. This was all in perfect agreement with existing theories of the early universe.

Liquid crystal polymers - the new plastics
	A thread that refuses to stretch Aramid fibre (left), developed by Hoechst, can support up to 50 per cent of its breaking load without stretching. It can also withstand temperatures of up to 450oC (below)
		Material that won't wear out In yet another manifestation of liquid crystals, Kevlar, manufactured by Du Pont, is used wherever great abrasion- resistance is required - such as in yacht-sails	Armour that resists a speeding bullet The ability of liquid crystals to withstand extreme pressures means that Kevlar is also one of the most commonly used materials for bullet proof vests

But the experiment did more than confirm the broad ideas. Counting the number of these defects, Dr Bowick and his colleagues compared what they saw with the predicted number from the theory used by cosmologists. It turned out that the number of defects they saw for each separate region also corresponded perfectly with theory.

Cosmologists, it seems, really might have a good understanding of the early universe after all. And who would have thought that those little black numbers silently changing in your watch or CD player could cast light on the mysteries of the cosmos?
Robert Matthews

INDEX

Nov 1994