Characterizing Complex Fluids
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Characterizing
Complex Fluids
L.J. Magid and P. Schurtenberger Abstract Among the experimental techniques used to characterize complex fluids, neutron scattering has played a unique and successful role, primarily for two reasons: (1) neutrons access the proper length and time scales, especially small-angle neutron scattering and reflectometry for structural and kinetic studies and neutron spin echo for dynamic investigations; and (2) for hydrogen-containing substances, the exchange of hydrogen by deuterium facilitates labeling on a molecular scale, an extremely important method for deciphering complex structures in multicomponent materials. In this short review, we give a number of examples for successful neutron studies of dense particle suspensions, including aggregation phenomena, in situ kinetic studies on shape transformations, shear-induced surfactant self-assembly phenomena near surfaces, and dynamics of complex fluids. Finally, we give an outlook on future developments. Keywords: colloidal materials, complex fluids, diffusion, layered structures, microstructure, neutron scattering, phase transformation, polymers, soft condensed matter, surface chemistry.
Introduction Complex fluids (often also called soft condensed matter) represent a rapidly expanding field of research in which one primarily focuses on three different complementary areas: colloids, polymers, and surfactants.1 A major goal is to understand the formation processes, structure, and functional properties of supramolecular systems that play an important role in everyday life. As the name implies, in soft condensed-matter research, one studies materials which are soft, that is, easily deformable by external stresses or even thermal fluctuations. Soft condensedmatter science is not only an attractive area of modern basic research, but is also of considerable technological importance in areas such as the manufacturing of synthetic dispersions for coatings, ceramics fabrication, polymer processing, corrosion phenomena, environmental pollution, food technology, the pharmaceutical industry, biocompatible materials, and biotechnology. The properties of complex fluids such as colloidal suspensions, microgels, liquid crystals, micellar solutions, microemulsions, and emulsions combine the features of classical solids and liquids in varying proportions. Viscoelasticity is commonly
MRS BULLETIN/DECEMBER 2003
observed in complex fluids: they are solidlike at short observation times and liquidlike at long times. An important feature of complex fluids is the extremely large range of characteristic length scales (0.1–1000 nm) and time scales (10–12 s to 10 s) that need to be covered in any attempt to characterize and understand their properties. This is illustrated in Figure 1, which compares their characteristic mass, lengths, and time scales (note that only time scales required for center of mass diffusion are considered, and local dynamics on the molecular scale has not been included) and specific surface areas with those of atomic and macroscopic syste
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