Peter S. Pershan

Polarized Raman scattering studies of orientational order in uniaxial liquid crystalline phases

The measurement of vibrational Raman depolarization ratios has been used to study molecular orientational order in uniaxial single domain nematic and smectic liquid crystal samples. This technique is demonstrated to obtain the same microscopic order parameter 〈P2〉=1/2 〈3 cos2ϑ−1〉, where ϑ is the angle between a molecular long axis and the uniaxial direction, as other existing methods. In addition, the next higher moment of the orientational distribution function 〈P4〉=1/8 〈35 cos4 ϑ−30 cos2ϑ+3〉 has been measured for the first time. The physical basis, theoretical apparatus, and experimental methods necessary for the application of this technique are thoroughly detailed in this paper. Measurements are presented of the temperature dependence of 〈P2〉 and 〈P4〉 of N‐ (p′‐butoxybenzylidene) ‐p‐cyanoaniline (BBCA) dissolved in N‐ (p′‐methoxybenzylidene) ‐p‐cyanoaniline (MBBA) and of pure MBBA in the isotropic and nematic phases, and in the isotropic, nematic, smectic A, and smectic B phases of N‐ (p′‐butoxybenzyl...

Structure of liquid crystal phases

Current understanding of different phases as well as the phase transitions between them has only been achieved following recent theoretical advances on the effects of dimensionality in statistical physics. P S Pershan explains the connection between these two separate areas and gives some examples of problems where the understanding is still not complete. The most important example is the second order phase transition between the nematic and smectic-A phase. Others include the relation between the several hexatic phases that have been observed and the first order restacking transitions between phases that were all previously identified as smectic-B, but which should more properly be identified as crystalline-B. Some relatively recent experimental developments on the discotic phase, liquid crystal surfaces and lyotropic phases are also included. The book includes 41 major reprints of some of the recent seminal work on the structure of liquid crystals. They are introduced by a brief review of the symmetries and other properties of liquid crystalline phases. In addition, there is a discussion of the differences between true liquid crystalline phases and others that were described as liquid crystalline in the early literature, but which have since been shown to be true three-dimensional crystals. The progression from the isotropic fluid, through the nematic, smectic, and various crystalline phases can be understood in terms of a systematic decrease in symmetry, together with an accompanying variation in structure is explained. A guide to the selected reprints and a sort of “Rosetta Stone” for these various phases is provided. The goal of this book is to explain the systematics of this progression to students and others that are new to this field, as well as to provide a useful handbook for people already working in the field.

Magneto‐Optical Effects

The general theory of magneto‐optical effects is reviewed and it is shown that one can describe these phenomena in terms of a macroscopic dielectric tensor by setting μ=1. This is true even when the microscopic origins of the dielectric constant are magnetic‐dipole transitions. For the case of electric‐dipole transitions, the dispersive magneto‐optical effects (i.e., Faraday or Voigt effects) can be interpreted in terms of an effective Hamiltonian which is related to the macroscopic dielectric tensor in a very simple manner. The form of this effective Hamiltonian follows from symmetry considerations. From the measurements of the magneto‐optical effects, one determines the constants of the effective Hamiltonian and from these one can predict the cross section for Raman scattering of light by spin excitations.

Surface Crystallization in a Liquid AuSi Alloy

X-ray measurements reveal a crystalline monolayer at the surface of the eutectic liquid Au82Si18, at temperatures above the alloys melting point. Surface-induced atomic layering, the hallmark of liquid metals, is also found below the crystalline monolayer. The layering depth, however, is threefold greater than that of all liquid metals studied to date. The crystallinity of the surface monolayer is notable, considering that AuSi does not form stable bulk crystalline phases at any concentration and temperature and that no crystalline surface phase has been detected thus far in any pure liquid metal or nondilute alloy. These results are discussed in relation to recently suggested models of amorphous alloys.

Flexoelectricity in Nematic and Smectic-A Liquid Crystals

Flexoelectric effects are observed in both the nematic and smectic‐A phases of p‐butoxybenzal‐p‐ (β‐methylbutyl) aniline (BBMBA) and p‐cyano‐benzylidine‐p‐octyloxyaniline (CBOOA). This is the first reported observation of flexoelectricity in smectic phases. The use of a symmetric interdigital electrode in the homeotropic geometry facilitated the unambiguous separation of linear and quadratic electro‐optic effects. Both the interdigital electrodes and those liquid‐crystal deformations that are quadratic in the voltage act as optical diffraction gratings with a spacing that corresponds to the repeat distance d for adjacent electrodes. In contrast linear electro‐optic effects give rise to diffraction gratings with twice this spacing since adjacent electrodes have opposite voltages. Diffraction maxima due to the linear effects are halfway between the maxima due to the other effects. Using optical heterodyne detection, the intensity of the diffraction maxima believed to arise from the linear effect are indeed ...

Lyotropic liquid crystals

In mixtures of soap and water, the special properties of soap molecules cause them to form clusters—and clusters of clusters—in a variety of interesting geometrical shapes. Some of these aggregates are liquid crystals, known as lyotropic, which are quite different from thermotropic liquid crystals, the focus of most of the work described in this issue of PHYSICS TODAY. Lyotropic liquid crystals are now receiving a great deal of scientific and technological attention because of the way they reflect the unique properties of their constituent molecules.

X-ray grazing incidence diffraction from alkylsiloxane monolayers on silicon wafers

X‐ray reflection (both specular and off‐specular) and grazing incidence diffraction (GID) have been used to study the structure of alkylsiloxane monolayers (n‐C18H37SiO1.5) formed by self‐assembly from solution on silicon wafers. GID studies of complete monolayers reveal a single ring of scattering associated with the monolayer. The Lorentzian line shape of this ring indicates that the film is characterized by liquidlike order, with a typical translational correlation length of about 45 A. The thermal coefficient of expansion of the monolayer, as determined from the GID peak position, is approximately equal to the value for liquid n‐alkanes. Upon either heating or cooling, the monolayer correlation lengths decrease, suggesting that the differential thermal‐expansion coefficients of the film and substrate figure prominently in thermal changes of the molecular ordering. GID data for incomplete monolayers also reveal a single ring of scattering associated with the monolayer. While both the translational corr...

Liquid surfaces and interfaces : synchrotron X-ray methods

1. Introduction 2. Instrumentation 3. Theory of x-ray scattering from liquid surfaces 4. Experiments on liquid surfaces and interfaces.

Re-entrant Appearance of Phases in a Relaxed Langmuir Monolayer of Tetracosanoic Acid as Determined by X-Ray Scattering

The structure of the fully relaxed phases of a Langmuir monolayer of tetracosanoic acid is determined by x‐ray diffraction and reflection along an isotherm at ∼20.5 °C. Isotherms taken by allowing the surface pressure to stabilize between incremental compressions are seen to be qualitatively different from the constant‐rate nonrelaxed isotherms typically seen in the literature. At low densities the monolayer consists of an inhomogeneous film of islands of a crystalline (or hexatic) phase with molecular tilt ordering that is analogous to that of the smectic I liquid crystal. Small amounts of impurities (∼0.5% of the monolayer) account for the change in surface pressure with area in this region. Upon compression to the point that the free space between islands becomes negligible the film appears homogeneous. On further compression the time required for full relaxation becomes long (i.e., ∼ hours), the tilt angle of the molecular axis decreases and the x‐ray unit cell is compressed. Including this homogeneou...

Monodomain Samples of Dipalmitoyl Phosphatidylcholine with Varying Concentrations of Water and Other Ingredients

Methods are presented for the preparation of large monodomain phospholipid bilayer arrays containing variable amounts of water approaching the two-phase limit. The optical birefringence of these lamellar phases of dipalmitoyl phosphatidylcholine (DDPC) is measured over a range of temperature and water content, and phase transitions are observed. The techniques employed for pure DPPC and water are extended in order to produce macroscopically aligned samples containing varying concentrations of cholesterol, inorganic salts, antibiotics, and chlorophyll a. Polarization studies of the 670-nm band of chlorophyll a indicate macroscopic orientational order in the chromophore under the same conditions.