Optical Rheometry of Complex Fluids PDF

Preface

The use of optical methods to study the dynamics and structure of complex polymeric and colloidal liquids subject to external fields has a long history. The choice of an optical technique is normally motivated by the microstructural information it provides, its sensitivity, and dynamic range. A successful application of an optical measurement, however, will depend on many factors. First, the type of interaction of light with matter must be correctly chosen so that the desired microstructural information of a sample can be extracted. Once selected, the arrangement of optical elements required to perform the required measurement must be designed. This involves not only the selection of the elements themselves, but also their alignment. Finally, a proper interpretation of the observables will depend on one's ability to connect the measurement to the sample's microstructure.

The title of the book, " Optical Rheometry of Complex Fluids'," refers to the strong connection of the experimental methods that are presented to the field of rheology. Rheol-ogy refers to the study of deformation and orientation as a result of fluid flow, and one principal aim of this discipline is the development of constitutive equations that relate the macroscopic stress and velocity gradient tensors. A successful constitutive equation, however, will recognize the particular microstructure of a complex fluid, and it is here that optical methods have proven to be very important. The emphasis in this book is on the use of in situ measurements where the dynamics and structure are measured in the presence of an external field. In this manner, the connection between the microstructural response and macroscopic observables, such as stress and fluid motion can be effectively established. Although many of the examples used in the book involve the application of flow, the use of these techniques is appropriate whenever an external field is applied. For that reason, examples are also included for the case of electric and magnetic fields.

This book has been written for the practitioner, as well as researchers seeking to either predict the optical response of complex liquids or to interpret optical data in terms of microstructural attributes. For these purposes, the book is meant to be self contained, beginning with sections on the fundamental Maxwell field equations describing the interaction of electromagnetic waves with anisotropic media. These interactions include

transmission, reflection, and scattering and are covered in the first four chapters of the book. Spectroscopic interactions, such as absorption, Raman scattering, and fluorescence are discussed in Chapter 5. Since complex fluids subject to external fields possess anisotropic optical properties, a great deal of attention is devoted to the effects of light polarization. Although the majority of the book is devoted to techniques for the measurement of dynamics and structure in the presence of external fields, methods of measurement of flow field kinematics are also presented. These are discussed in Chapter 6 and include dynamic light scattering and laser Doppler velocimetry.

The connection between the observables extracted from optical measurements, and the microstructure of polymeric and colloidal liquids is presented in Chapter 6. This is developed in terms of current models of molecular and particulate dynamics. The study of the dynamics and structure of complex liquids is interdisciplinary, involving physicists, chemists, and chemical engineers. Recognition of this wide audience is reflected in the applications that are included, where examples are drawn from each segment of the community.

The design of an optical instrument must include an analysis of the detected signal. For many problems, this can be conveniently accomplished using Mueller calculus. The methodology of this procedure is explained, and matrices are developed for most optical elements encountered in the laboratory. This treatment is offered at a sufficiently general level so that the reader is supplied with the methods to generate Jones or Mueller matrices for complex, composite optical elements. The particular optical arrangement that is chosen must meet many requirements, and these are presented in Chapter 8. Most important, sufficient information must be extracted to isolate the desired optical properties of the sample. In addition, the timescale of the measurement must be commensurate with the response time of the sample if a time-dependent field is applied. For this purpose, polarization modulation and wavelength modulation techniques are discussed. These considerations are used to guide the design of polarimeters for the measurement of birefringence and dichro-ism, scattering experiments for measurement of the structure factor, and spectroscopic schemes capable of extracting the dynamics of individual components in complex mixtures.

The success of an optical measurement will be controlled by the quality of the optical components that make up the instrument, and the accuracy of their alignment. For this reason, Chapter 9 is devoted to the selection of specific components to accomplish a required task. Since such a decision is influenced by the construction of an element, the underlying physics and design criteria used in the manufacture of optical components are presented. This discussion is combined with alignment protocols that the author and his students have found useful in their own laboratory.

The final chapter on applications of optical rheometric methods brings together examples of their use to solve a wide variety of physical problems. A partial list includes the use of birefringence to measure spatially resolved stress fields in non-Newtonian flows, the isolation of component dynamics in polymer/polymer blends using spectroscopic methods, the measurement of the structure factor in systems subject to field-induced phase separation, the measurement of structure in dense colloidal dispersions, and the dynamics of liquid crystals under flow.