The fascinating world of liquid crystals
While liquid crystals to the broad public are probably best known for their use in flat panel displays (LCDs), present-day academic liquid crystal research focuses very much also on totally different aspects of these fascinating ordered fluids. The common characteristic of all liquid crystal phases (more than a hundred have been identified to date) is the counterintuitive combination of fluid-like molecular mobility and long-range order. Orientational order is always present (with the common direction of the constituents being indicated by a pseudo-vector called the director) and translational order often appears as well, the latter being of 1-, 2- or 3-dimensional type (in the 3D case the order is however only short-range in at least one of the three dimensions). Among the translationally ordered liquid crystals a rich subdivision of generic classes into different phases, with subtle differences in terms of molecular organization, leads to a broad spectrum of symmetries with important consequences for their physical properties. Chiral liquid crystals display a particularly rich morphology, with fascinating physical properties such as narrow band-gap reflection of circularly polarized light or ferro- and antiferroelectricity.
The materials forming liquid crystal phases range from the half-rigid–half-flexible organic molecules used in LCDs, to colloidal suspensions of disc- or rod-like nanoparticles (organic—e.g. viruses—or inorganic, e.g. clay or carbon nanotubes), to aqueous suspensions of amphiphilic molecules such as surfactants or phospholipids. A most important example of the latter class is encountered in our own bodies: the lamellar liquid crystal phase is the natural state of our phospholipid-based cell membranes. This is vital for the correct functioning of the cell, as it provides the membrane with just the right combination of rigidity and flexibility that life requires.
The structural features of the translationally ordered liquid crystal phases have characteristic length scales that vary from nanometers to micrometers, depending on if the features are only molecular (nanometer scale)—such as lamellar or columnar molecule arrangements—or if also supramolecular structures (micrometer scale), like the helical modulations that are characteristic of chiral liquid crystals, are present. The correlation length of orientational as well as translational order is typically on the order of hundreds of micrometers. Both types of order are a result of self-assembly: with the right temperature, pressure and composition (in many cases a single component is actually enough), the particular structure that is inherent of the phase in question will arise spontaneously. In addition, liquid crystals offer us, as described in the following, a great means of controlling the self-assembly process, which is dynamic and always on-going.
Being anisotropic fluids, liquid crystals easily and strongly respond to various types of external influence such as electric or magnetic fields or the presence of surfaces with particular properties. We can thus often extend the liquid crystalline order over macroscopic distances, and in many cases we can align the structure along directions of our choice. Moreover, since the structure appears in a liquid phase, it is generally possible to get the system to rearrange by simply applying a new external field, resulting in a change of the physical properties. The response can be as fast as microseconds and strong enough to be applicable in a whole range of applications. The best known example is of course the liquid crystal displays, where a small applied voltage rearranges the liquid crystal from the field-free state that the bounding substrates induce, thereby changing the optical properties of a display element. On removing the electric field, the liquid crystal relaxes back to the substrate-induced ground state. In the words of Nobel laureate Pierre Gilles de Gennes, the unique combination of self-assembled order and ease of dynamic external control gives liquid crystals a giant response function, making them prime examples of the materials class that today is referred to as soft matter.
Due to their rich structural variety, spanning very different length scales and symmetries, liquid crystals are today attracting great interest in diverse areas of nanotechnology and materials science. They are finding new uses e.g. as versatile and dynamic templates for synthesizing 3D-nanostructured materials that would be impossible to obtain with other methods, or for aligning anisometric nanoparticles such as carbon nanotubes. Efforts are devoted to exploiting the self-organized micron-scale superstructures of chiral liquid crystals for photonic crystals and tunable lasers. The giant response function of liquid crystals is also the basis for their proposed use in a variety of sensors—in particular biosensors—as well as in photo- or thermoresponsive actuators, where liquid crystalline elastomers constitute the material of choice. In the field of pharmaceutical and food research, liquid crystalline nanoparticles are today being commercialized as vehicles for delayed drug / nutrient delivery and in biology and biotechnology the liquid crystallinity of DNA is gaining an increasing interest, with impact even on prebiotic chemistry and the origin of life. These examples—far from exhaustive—serve to illustrate that liquid crystal research is an extremely rich research field, embracing chemistry, physics, biology and various aspects of modern technology. New fascinating fields are continuously being opened, in particular where liquid crystals meet colloid and polymer science or bio- and nanotechnology.
A good short introduction to thermotropic liquid crystals, prepared by Prof. Lagerwall and his co-workers while he was a Ph.D. student, can be found at the Chalmers University of Technology Liquid Crystal Group web site. Although it is by now a bit dated, it gives a quick introduction to the key aspects of thermotropic liquid crystals, with a focus on chiral smectics and their electrooptic applications.
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