Colloidal crystals description

Table 4.2 Steps for the fabrication of the colloidal crystal films and for determination of selective chemical sensing response Step Description...

The above features of a sheared colloidal crystal appear to be similar in both BCC and FCC structures. However, there are differences in details, and perhaps even within a given symmetry the flow behavior might vary with particle concentration or charge density. For example, Chen et al. (1994) have shown that between the strained crystal and sliding-layer microstructures there can be a polycrystalline structure, the formation of which produces a discontinuous drop in shear stress (see Fig. 6-33). Ackerson and coworkers gave a detailed description of the fascinating shear-induced microstructures of these systems (Ackerson and Clark 1984 Ackerson et al. 1986 Chen et al. 1992, 1994). 

An important advantage of using colloidal crystals as nanoporous membranes is their highly ordered nature, which allows using accurate mathematical descriptions of the transport rate [21-25]. The effective diffusivity of molecules in the fee lattice Dfcc, can be expressed as (8/t)Dsoi, where D oi is the diffusivity of molecules in... 

The above picture of an easy mapping of the averaged trajectories of the colloids on a quasi-macroscopic description only held for amorphous structures, i.e., when the largest microscopic length scale was the colloidal diameter. In case the colloids formed some kind of structure, e.g., colloidal crystals or fractal structures, the situation was different. These internal structures brought in a different length scale that interfered with the averaging. We illustrate this in the case of a polycrystalline colloidal film [77]. 

Physical state and its description. The physical state of each substance is indicated, in the text and in column 2 of the table, as gaseous, liquid, crystal, glass, colloidal, or in aqueous or other solution. (See the list of abbreviations on page 14.) All states are for a pressure, or a fugacity, of one atmosphere and a temperature of 18°, unless otherwise indicated. 

Statistical mechanics was originally formulated to describe the properties of systems of identical particles such as atoms or small molecules. However, many materials of industrial and commercial importance do not fit neatly into this framework. For example, the particles in a colloidal suspension are never strictly identical to one another, but have a range of radii (and possibly surface charges, shapes, etc.). This dependence of the particle properties on one or more continuous parameters is known as polydispersity. One can regard a polydisperse fluid as a mixture of an infinite number of distinct particle species. If we label each species according to the value of its polydisperse attribute, a, the state of a polydisperse system entails specification of a density distribution p(a), rather than a finite number of density variables. It is usual to identify two distinct types of polydispersity variable and fixed. Variable polydispersity pertains to systems such as ionic micelles or oil-water emulsions, where the degree of polydispersity (as measured by the form of p(a)) can change under the influence of external factors. A more common situation is fixed polydispersity, appropriate for the description of systems such as colloidal dispersions, liquid crystals, and polymers. Here the form of p(cr) is determined by the synthesis of the fluid. 

Size-dependent structure and properties of Earth materials impact the geological processes they participate in. This topic has not been fully explored to date. Chapters in this volume contain descriptions of the inorganic and biological processes by which nanoparticles form, information about the distribution of nanoparticles in the atmosphere, aqueous environments, and soils, discussion of the impact of size on nanoparticle structure, thermodynamics, and reaction kinetics, consideration of the nature of the smallest nanoparticles and molecular clusters, pathways for crystal growth and colloid formation, analysis of the size-dependence of phase stability and magnetic properties, and descriptions of methods for the study of nanoparticles. These questions are explored through both theoretical and experimental approaches. 

Double layers are also important in colloid chemistry. When a colloid particle is composed of an ionic crystal, it often preferentially adsorbs one of its component ions, thereby acquiring a charge. As a result the colloid particle is surrounded by a double layer. The interfacial properties are very important in determining a variety of colloidal properties, including electrophoresis and electroosmosis. It also plays a role in colloid stability and coagulation phenomena. The effects of the electrical properties of the interface are well known in colloid chemistry. The description of colloid phenomena is a well-developed area of physical chemistry which is often important in industrial processes. 

Yet another development of remarkable nanostmctured materials yields superlattices of nanosized objects. As there is no dear distinction between molecular crystals and superlattices formed from nanopartides, at this point reference will be made to structures composed of very similar (but most likely not exactly identical) nanopartides, namely colloidal partides in the size range 2 to 10 nm. Two excellent reviews by leading experts in the field were produced in 1998 and 2000 [19, 20], the titles of which contained the terms nanocrystal superlattices and close-packed nanociystal assemblies. These are in line with the above-outlined delimitation, although Collier et al. have also reported on molecular crystals (as above). The two reviews comprised approximately 100 pages with some 300 references, and summarized the state of the art at that time in exemplary fashion. The topics induded preparative aspects of the formation of monodisperse nanopartides of various compositions including metals, the superlattice formation itself with some theoretical background, covalent linking of nanocrystals (see below), and an appropriate description of the physical properties and characterization of the nanocrystal superlattices. 

The following is restricted to membrane phases. However, the description is general enough that it stands for soft matter systems that possess internal disorder (liquid crystals, colloids, polymers, plastic crystals, etc.)... 

Defects can also be trapped by an object submerged within the liquid crystal. For example, Terentjev [29] gave a theoretical description of a -1/2 disclination line bound just outside the surface of a colloidal particle in a nematic fluid, as illustrated in Fig. 11. Here, also, one can imagine an applied field changing the equilibrium loop size. The response depends not only on viscosity but also on the elastic constants and the defect core energy. 

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