Liquid Crystals: Experimental Study of Physical Properties and Phase Transitions

Edited by:
  • Satyendra Kumar
Cambridge University Press: 2000. 483 pp. £70, $110

Any discussion of liquid-crystalline phases usually starts by describing them as a state of matter that is intermediate in structure between the more-or-less totally ordered crystalline solid phase and the completely disordered state of a liquid. This is a perfectly valid statement, but it carries all the wrong connotations: it implies that the properties of these 'mesophases' will also be intermediate. Admittedly, in some cases this is true. But in general, the combination of fluidity and spontaneous molecular ordering gives rise to unique and remarkable phenomena.

The liquid-crystal phases have, with good reason, been called nature's 'sensitive' phases. In fact, they are sensitive to virtually everything. Some change colour with temperature (and are sensitive to one-hundredth of a degree). Others respond to small electric fields and are readily aligned on treated surfaces, a combination of properties that makes them uniquely useful for display devices. Others form solvent phases that can align dissolved solute molecules, and this phenomenon is utilized in display devices and in new forms of spectroscopy. It is also possible to produce highly ordered, ultra-strong materials, notably Kevlar, by spinning fibres from liquid-crystalline polymer melts (rather than disordered liquids).

These unique characteristics are, of course, an expression of measurable physical properties, and over the past 20 years or so techniques for measuring them have featured increasingly in the research literature. The “long felt need” identified by Satyendra Kumar “for a source of general, non-superfluous and practical information regarding the experimental research techniques and how they are applied to liquid crystals” is real enough, and the book he has put together is most welcome.

Fluid functions: liquid crystals examined by light (inset) and polarizing microscopy. Credit: JAMES BELL/SPL

Liquid Crystals competently and elegantly covers the principal techniques used to investigate liquid crystals — these include polarizing microscopy, differential scanning calorimetry, X-ray diffraction, nuclear magnetic resonance and light scattering. And the chapter on physical properties covers all the familiar themes — dielectric properties, optical diamagnetic elastic and viscous properties, electrical and thermal conductance and density measurement — with 20 pages of colour plates showing the main optical textures. There is a remarkable chapter on freely suspended films — films stretched across a hole in a plate so that they are not in contact with the solid surface on either side. And the book ends with a discussion of the relationship between chemical structure and mesogenic properties. Each topic receives a mathematical treatment, at a level comprehensible to a physics graduate.

The patchwork of chapters by a dozen authors is almost inevitably selective and idiosyncratic, and in many places the book has depth but not breadth. Esoteric points are sometimes discussed in detail. For example, the authors from Ohio's Kent State University are proud of their characterization of the elusive biaxial nematic phase and could not resist mentioning it in the introduction. But there is virtually nothing about lyotropic phases, polymer liquid crystals or discotics. Nevertheless, Kumar has achieved his goal of producing a volume of practical information with “in-depth technical details”. For students about to embark on experimental investigations of thermotropic, nematic or smectic systems, this book could be one of their best buys.