Fourth state of matter: between solid and liquid
What is a liquid crystal?
When you think about it, it is surprising to hear someone talk about liquid crystals. It is not easy to put the two words together when you consider what each of them means. However, although their knowledge is still somewhat limited, it is known that they owe their properties to the characteristic of some chemical compounds to present this fourth state of aggregation of matter.
The first to observe a liquid crystal was the Austrian botanist Friedrich Reinidzer in 1888, when he saw that cholesteryl benzoate (a solid substance derived from cholesterol) formed a cloudy liquid when heated to its melting temperature. On further heating, the turbidity persisted until at a certain temperature the liquid became clear. Shortly afterwards, other substances were discovered that exhibited the same behaviour, and it was soon shown that this new state of matter appeared to be intermediate between solid and liquid.
It is precisely in this special characteristic that its interest lies: it combines certain characteristics of crystalline solids, showing different properties in different directions, together with certain properties of liquids such as mobility and fluidity. For this reason, in 1889 the German physicist Otto Lehmann called them “Liquid Crystals”, the name by which they are still known today.
Liquid crystals in nature
Liquid crystals are not the exclusive product of our technology. As in other areas of science, nature is the first school where we can learn about these unique compounds. Liquid crystal-type organisers exist in many biological systems. One of the best known examples are the so-called phospholipids, the main component of cell membranes. Another example is myelin fibres, a lipoprotein found coating the axon of neurons. In the same context we can also mention the liquid crystals formed by products such as carbohydrates, polypeptides and nucleic acids.
What does a liquid crystal look like?
In a crystalline solid, the molecules occupy fixed positions and are oriented in a specific way with respect to each other. This means that some of their properties change depending on the direction of the space under consideration: this is called anisotropy (for example, a mica crystal can be easily exfoliated in the direction of its constituent lamellae, but not in the perpendicular direction). On the contrary, in a liquid, the molecules are completely disordered, which gives it its characteristic fluidity (capacity to adopt the shape of the container that contains it) and its properties are isotropic (they do not depend on the direction considered). Liquid crystals combine the ease of movement of liquids with the anisotropy of solids. This makes them unique for certain purposes.
This anisotropy is responsible for the characteristic textures observed in thin layers of a liquid crystal under an optical microscope between crossed polarisers, which depend on the way their molecules are arranged and oriented.
They are compounds formed, in general, by molecules with anisotropic dimensions. Some liquid crystals are formed by rod-shaped molecules (Calmitic Liquid Crystals). Others are made up of disc-shaped molecules (Discotic or Columnar Liquid Crystals). These two are the main types of molecules that give rise to the appearance of the liquid crystal state due to the effect of temperature (thermotropic liquid crystals). There are other types of liquid crystals, the lyotropic liquid crystals that appear due to the presence of a solvent. The most typical structure of the molecules that form them consists of a hydrophobic and a hydrophilic part within the same molecule; the solvent may be water or another less polar solvent that induces the formation of micelles, which in turn are arranged in the liquid crystal state.
Applications of liquid crystals
Liquid crystals, depending on their properties, can be used for different purposes. For example, those that reflect light of different colours depending on the temperature are used in thermometers or detectors of tumours or cracks in mechanical surfaces. Because of their electro-optical properties, they are used as the basis for television screens, computer monitors, video projectors, printer heads, calculator screens, watches or electronic games, etc. As light valves, they are capable of accepting an image of low light intensity and converting it into a more intense output.
Finally, lyotropic liquid crystals are currently of great importance to the detergent and cosmetics industries.
Optical devices
One of the main reasons for industrial interest in liquid crystals has been their use in the design of devices that combine the fluidity with the optical and dielectric anisotropy of these materials, the so-called “displays”.
In a watch, a calculator or an electronic game, ambient light reaching the surface of the display passes through the different components of the device, which are placed in a sandwich structure. But this light will be reflected back if the orientation of the molecules of the liquid crystal layer in the centre of the display is appropriate. This orientation can be controlled locally by switching on or off an electrical circuit set up with the small voltage of a battery. The result is that differentiated areas of brightness or colour appear on the screen, so that we see a particular number or figure. One of the key advantages is that they can change in a matter of milliseconds or microseconds.
Ultra-strong fibres
One of the most representative cases is Kevlar, a polymer with exceptional mechanical properties, superior to those of steel in terms of property-to-weight ratio. The surprising properties of Kevlar are based on obtaining fibres from a polyamide in sulphuric acid when a liquid crystal phase of the lyotropic type has been formed. In this state, the molecules interact with each other and organise themselves in a perfect longitudinal orientation, an arrangement that is maintained after the sulphuric acid has been removed and the fibre is ready to work with. This high organisation along the radius and axis of the fibre is ultimately responsible for its excellent tensile properties, which has enabled it to compete in the manufacture of tyres, cables, ropes, bulletproof vests, military helmets, protective gloves, etc.
Biological applications
Micrograph of a liquid crystal
Cell membranes are made up of liquid crystals. These biological organisations may have applications in other fields. For example, liposomes are hollow spheres with a liquid crystal structure made of phospholipids or other lipid derivatives. Inside these spheres, it is possible to store active ingredients that can be released little by little. Cosmetics is one of the fields where these compounds are used.
¿Qué hacemos en el INMA?
Micrografía de un cristal líquido.
El INMA se ha dedicado al diseño, síntesis y caracterización de cristales líquidos orgánicos con mejores y nuevas propiedades no sólo ópticas sino también eléctricas o magnéticas. Por el carácter interdisciplinar del tema, el grupo formado por químicos y físicos del INMA y colabora con otros científicos de universidades españolas y extranjeras, así como con equipos de empresas europeas como Philips y Merck, en la búsqueda de nuevos materiales y su posterior aplicación.
También investigamos en nuevas propiedades y aplicaciones de los cristales líquidos. Así por ejemplo, se está estudiando la aplicación de un tipo especial de polímeros cristal líquido para grabación óptica de datos, como sistemas de grabación alternativos a los CDs o DVDs actuales. La capacida de grabación de estos sistemas holográficos es impresionante y se prevé que un solo disco pueda almacenarse una información superior a la disponible en 1000 CDs actuales.
Otras líneas de trabajo están dirigidas hacia la utilización del estado de cristal líquido como herramienta para conseguir nuevos tipos de materiales con mejores propiedades. El cristal líquido se caracteriza por el orden de las moléculas que lo forman. Controlando este orden en los materiales que preparamos, podremos mejorar las propiedades e incluso conseguir varias propiedades en el mismo material (por ejemplo, combinando propiedades eléctricas y ópticas). Conseguimos así materiales con múltiples funciones, “materiales multifuncionales”, para nuevas aplicaciones en electrónica, biomedicina, dispositivos ópticos, etc.
More information
Brochure about liquid crystals
Exhibition of liquid crystals
On the occasion of the 25th Anniversary of the ICMA an exhibition of photographs on liquid crystals was organised, here you can see some of the photographs.
Micrograph of a liquid crystal
Micrograph of a liquid crystal
II SOCIEMAT Materials School: Applications of liquid crystals.
In 2010, the 2nd Materials School was organised within the framework of the 11th National Materials Congress, organised by the Spanish Materials Society and our Institute. This edition of the School of Materials was intended to be a link between researchers in Materials Science and Technology and secondary education teachers to facilitate the introduction of teaching related to materials in secondary education. Applications of Liquid Crystals” was chosen as the theme.
In the first part of the School, researchers from our Institute explained to secondary school teachers the basics of some applications of these materials and then worked in a coordinated way between teachers and researchers to have panels and a bank of experiments to develop in the classroom.
The work topics chosen were:
Electro-optical devices (smart glasses and welding shields)
INMA researchers have prepared a video showing these materials, their applications, as well as the lines of work developed at INMA. It can be consulted here.
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