Creating crystal phenomena

Last year, Science carried a "Perspective" introducing two reports of exciting research on liquid crystals, including v-shaped molecules referred to as bent-core molecules or "bananas". One of the reports described a successful strategy for producing a ferroelectric phase from achiral bent-core molecules, the other explained how liquid crystalline phases form when bent-core molecules are mixed with a particular class of rod-like molecules.

Last month, Nature published a letter reporting experiments that demonstrated the non-equilibrium conformation of a side-chain liquid crystal polymer (SCLCP) close to the isotropic-nematic (I-N) transition, by showing that parallel or perpendicular orientation of the mesogens, with respect to the main chain, can be reversed by the application of an external field produced by shear flow. The paper was important because flow-induced phase transitions are a fundamental property of non-equilibrium systems, and are useful for tuning the processing conditions for plastics and petroleum products.

Suppose that you are interested in these phenomena and have some general knowledge of liquid crystals but are not familiar with their details. Where could you turn?

Satyendra Kumar, a specialist in X-ray diffraction studies of liquid crystals, has edited a symposium that is intended to give a survey of experimental studies of the physical properties and phase transitions of liquid crystals. After an introduction to liquid crystals, there are chapters on characterisation, X-ray diffraction, physical properties, NMR, light scattering and quasi-elastic spectroscopy, calorimetry, freely suspended film experiments, X-ray surface scattering and chemical structure-property relationships. There are 244 references and lots of tables. There is also a fantastic set of colour plates, mainly containing light micrographs, although the imaging conditions are not specified and there are no scale bars.

As with any symposium, the quality varies and continuity presents a challenge. The opening chapter is confused in the level of knowledge it assumes in the reader. Sometimes it seems like little more than a search engine: "readers are referred to the literature for more details on these phases". In other places, it delves into the esoteric as though wanting to bring in an isolated fact that the author cannot bear to prune: "it should be pointed out that, despite some reports, there are no known thermotropic biaxial nematic phases".

Polymer liquid crystals are mentioned in chapter one but not thereafter. The chapter opens with a very basic explanation of why the simple division into the three states of matter (solid, liquid and gas) is not the whole story, yet a few pages later we are invited to "recall that the chiral nematic phase is locally a nematic with a slow rotation of the director". But the director has not been defined and is defined only in a later chapter, which is not mentioned in chapter one (although this was jointly written by the editor). Another example is in the section describing "bownana" molecules (the authors' term for bow, banana, boomerang or bent-core molecules), where they refer to "sergeants-and-soldiers biasing of the global chirality upon addition of chiral dopants". But we are not told what "sergeants-and-soldiers" biasing is. Even the attempt to rationalise the classification of the smectic phases is only partially successful.

After this unpromising start, the chapters get better and better. Chapter four, on physical properties, is a model of good exposition. The terms and concepts are well set out and defined. The tensor algebra is clearly explained and there is just enough to enable the reader to relate second-rank tensor quantities in the lab coordinate system to those in the sample. This chapter covers dielectric, optical, diamagnetic, elastic and viscous properties, electrical and thermal conductance, and density measurements. There is ample reference to other chapters. I also enjoyed the chapter on freely suspended film experiments. Although mainly written with specific reference to X-ray scattering, it would apply to thin-film measurements of light scattering, torsional oscillations, electron diffraction or even heat capacity.

The editor believes the book should meet "a long-felt need for a source of general, non-superfluous and practical information regarding the various experimental research techniques and how they are applied to liquid crystals". On that basis, it should serve as a starting point for graduate students, postdoctoral research associates and scientists from other sub-fields of physics, chemistry and polymer science. It may also give established researchers insight into techniques beyond their expertise. There is plenty of mathematics to enable one to make sense of the concepts underpinning the experiments and to make quantitative deductions from experimental data. By and large, the volume succeeds in its goals and should be a valuable resource for anyone undertaking advanced research in liquid crystals.

Andrew Briggs is professor of materials, University of Oxford.