In the last issue of Liquid Crystals Today, I gave an overview about the recently published Handbook of Liquid Crystals and reviewed Volume 1, the first out of a compendium of eight books. In this issue, we shall review the second volume, which covers Volume 2: Physical Properties and Phase Behavior of Liquid Crystals. Obviously, this is another volume which is concerned with the more fundamental aspects of liquid crystals, laying the groundwork for more specialised topics to come. Accordingly, the volume is split into two parts: Part 1: Physical Properties, which takes up the majority of about 420 of the 520 pages (12 chapters), leaving about 100 pages for Part 2: Phase behavior, which consists of four chapters.
In chapter 1, David Dunmur and Geoffrey Luckhurst introduce the tensor properties of anisotropic materials, together with an introduction of the order parameters. The description covers the uniaxial as well as the biaxial nematic phase, all the way from the scalar-order parameters, the Q-tensor description, to the Wigner matrices. The main experimental methods to determine the order parameters are discussed, and a subchapter about chiral liquid crystals is included. Chapter 1 only covers orientational-order parameters though positional order of smectic liquid crystals or bond-orientational order of hexatic phases is not described.
Chapter 2 gives an overview of the magnetic properties of liquid crystals, written by David Dunmur, Malgosia Kaczmarek and Tim Sluckin. Different types of magnetism are discussed, as is the magnetic Freedericksz effect. For several molecular cores, tables are presented with the diamagnetic anisotropy. This is helpful, as is the discussion of paramagnetic metal containing mesogens and lanthanides. Also ferronematics are mentioned, and the future will tell if this class of materials may be observed more often than it is now.
It was also David Dunmur who wrote the following chapter 3 on the optical properties of liquid crystals. Besides the refractive indices, birefringence and the indicatrics, the molecular theory of refractive indices is presented, relating the molecular polarisabilities to the refractive indices, again illustrated with a useful table of different standard mesogens. Personally, I think that it would have been nice to see a more elaborated description of refractive index dispersion, as this is often neglected over the visible range of the spectrum. Optical absorption and linear dichroism are treated, and properties at IR and THz frequencies are briefly commented on. This is followed by a discussion of the optical properties in relation to phase structure and helical phases.
In chapter 4, Rosario de la Fuente and David Dunmur discuss the dielectric properties of liquid crystals. Starting with the dielectric response of an isotropic liquid, they introduce relaxation behaviour, the Debye equations and the Cole-Cole plot, the Cole-Davidson and the Havriliak–Negami equation. The dielectric properties of anisotropic fluids are discussed followed by the relaxation behaviour of a nematic, that is, rotations around the long and the short molecular axes. Molecular interactions and flexibility are then accounted for. Examples are provided for the nematic phase, bent-core nematics and dimers. Especially the ferroelectric and antiferroelectric chiral smectic C* phases are then discussed in much detail with a discussion of the collective modes, the Goldstone and the Soft-mode. Additional examples are provided for the complete temperature behaviour through the fluid smectic phases, including the intermediate phases, as well as the smectic and columnar phases formed by bent-core mesogens.
The elastic and flexoelectric properties are introduced in chapter 5, again by David Dunmur. Starting from Hooks law, the stress and strain tensors are introduced and the elastic free energy density written in terms of the principle elastic distortions splay, twist and bend, as common for the nematic phase (saddle splay is also mentioned for curved surfaces, for example, in lyotropics and for membranes and shortly discussed later in the chapter). Elasticity theory of smectic phases is briefly touched on, but a full description, for example, for SmC* is not elaborated. The chapter continues with a treatment of elasticity for non-uniform director distributions, (point) defects and director fluctuations. Flexoelectricity is introduced on a basic level, and it would have been beneficial to go into a bit more detail, for example discussing flexoelectric coefficients in terms of molecular shape anisotropy and by providing some exemplary experimental measurements. Nevertheless, the following subchapters discuss electric and magnetic field-induced deformations, used as methods to determine elastic constants. Specifically, the Freedericksz transitions are mentioned: the capacitance and the birefringence methods.
The chapter on defects and textures (chapter 6) was written by Oleg Lavrentovich. After a brief, but nice concise, introduction to polarising microscopy and fluorescence confocal polarising microscopy, many examples of defects and the most standard textures are provided. The main focus is not so much the characterisation of different phases, which had already been given in volume 1, but the occurrence and behaviour of defects. Droplets with their boojums and hedgehog defects are discussed: topological charges of defects, singular disclinations, schlieren defects and the Volterra process. This is followed by chiral nematics, with their oily streaks, disclinations and dislocations, double twist and the Blue Phases. For smectic textures and defects, batonnets are investigated: focal conics, dislocations and grain boundaries. Overall, it is an interesting chapter, which benefits from colour reproduction of the textures and defects.
The viscosity of a liquid crystal is obviously of great importance for applications, and Herbert Kneppe and Frank Schneider devote chapter 7 to this topic. They discuss shear viscosity coefficients and the rotational viscosity and outline different methods for their experimental determination. Their temperature dependence is shown from measured curves and the influence of molecular structure on the viscosity is discussed. Pretransitional effects at phase transitions are also pointed out.
Lev Blinov authored chapter 8 on electric field effects in nematics and chiral nematics. Some of these effects have partially been touched on in previous chapters, but it is useful to have the discussion condensed into one chapter. The Freedericksz transition is described, together with the threshold behaviour and the switching dynamics. Backflow phenomena find a mention, as do field-induced periodic structures and effects of surface bistability. The description of flexoelectricity also includes instabilities. For cholesterics, helix unwinding is outlined, again with the threshold behaviour derived. The flexoelectrooptic effect of short pitch cholesterics is schematically explained, but unfortunately no experimental behaviour is provided.
There are no applications of liquid crystals without alignment (given a few exceptions). Benjamin Broughton provides some insights into the surface alignment of liquid crystals in chapter 9. Depending on application, different alignments may be used: Freedericksz cell, twisted nematic, supertwisted nematic hybrid aligned cells or Pi cells, just to name a few. Methods of producing alignment are presented, from simple rubbing to micropatterning, in situ polymerisation and UV alignment. Switchable alignment with active alignment layers rounds off this chapter.
The optical properties of liquid crystals had been discussed before. Chapter 10, written by Istvan Janossy, discusses nonlinear optical properties, such as harmonic generation, nonlinear refraction, optical reorientation, azo-functionalised LC side-group polymers, light-induced surface realignment, nonlinear waveguiding and photo-induced structural changes. As interesting as these effects actually are, in my opinion this chapter would have benefitted from more experimental results and illustrations.
Chapter 11 by Kristiaan Neyts and Filip Beunis describes ion transport in liquid crystals. Commercial liquid crystals are laboriously cleaned from ionic contamination, because even small amounts of positive or negative ions can have pronounced effects on the performance of liquid crystals. It is thus more than adequate to include a chapter on this aspect. At first, the effects of ions on the performance of liquid crystal devices are discussed, such as the voltage holding ratio, image sticking and electro-convection. The underlying physics of these effects, transport of ions via electric fields or thermal motion (ion drift and diffusion) is then modelled.
Philippe Barois gives an overview of the theories of phase transitions in chapter 12. Liquid crystals are a very rich field of first- and second-order phase transitions, so a chapter of this kind should of course not be missing. In terms of the Landau approach, at first, the isotropic to nematic transition is discussed for both uniaxial and biaxial nematics. The nematic to smectic A transition follows with the definition of a smectic order parameter and the mean field description following McMillan and de Gennes. Analogies to superconductors are pointed out, as are critical exponents and scaling. The smectic A to smectic C transition is of special interest, but also transitions to hexatic phases are described. Phase transitions in chiral materials give rise to the twist grain boundary (TGB) phases, which are briefly introduced via their structure, followed by a discussion of frustrated and polar smectics, providing an overview of models and mean field phase diagrams.
Coming to Part II, the phase behaviour of liquid crystals is demonstrated with a wealth of experimental data. Chapter 13 by Cheng-Cher Huang illustrates phase transitions in liquid crystals via thermal studies. It should be noted that this chapter does not cover standard differential scanning calorimetry (DSC), but solely AC calorimetry. Discussed are the smectic A to smectic C and the smectic A to smectic B transition. I believe that it would have been instructive to give a short account of DSC measurements, especially also for compounds with a rich polymorphism, as this is the standard technique mainly used by liquid crystal researchers. This could have then been followed by the more sophisticated techniques on individual transitions, where also the isotropic to cholesteric (via Blue Phases) transition would have been of interest, as well as liquid crystal transitions in confined geometries.
In chapter 14, Daniel Guillon gives an account of the density of liquid crystals. In fact, these are measurements that are not often carried out to the required precision, and the density of a liquid crystal is often set equal to that of water. Nevertheless, the density can be related to phase transitions, pretransitional effect and structural parameters, the latter in combination with X-ray experiments.
Not only temperature as a thermodynamic variable of state can reveal phase transitions, also pressure can, as outlined by the high-pressure investigations summarised by Stanislaw Urban in chapter 15. Typical phase diagrams T(p) are shown for calamitic mesogens, as well as bent-core materials, discotics and ferroelectrics. Also pressure-volume-temperature measurements are discussed, as are effects on order parameters and dielectric behaviour.
Following is the last chapter of this volume 2 of the new Handbook of Liquid Crystals, chapter 16 on re-entrant phase transitions, written by Patricia Cladis, Richard Mandle and John Goodby. Re-entrant behaviour from frustration, from geometric complexity and from competing fluctuations is discussed, and a whole range of experimental phase diagrams presented, illustrating re-entrant nematic and TGB A phases and discussing the interesting sequence involving re-entrant isotropic behaviour. At last, structure–property correlations are pointed out.
Like the previous volume discussed, also volume 2 is completely revised and enlarged, with the exception of only very few chapters. Most of the times experimental results are presented where appropriate, and the references are updated without neglecting important papers of the past. The individual chapters are self-contained and can be read without prior knowledge of all the other chapters of the volume. Inevitable overlap between chapters is kept to a minimum. It can be hoped that individual volumes will be made available by the publisher in the future.