Colloids nonspherical

Any study of colloidal crystals requires the preparation of monodisperse colloidal particles that are uniform in size, shape, composition, and surface properties. Monodisperse spherical colloids of various sizes, composition, and surface properties have been prepared via numerous synthetic strategies [67]. However, the direct preparation of crystal phases from spherical particles usually leads to a rather limited set of close-packed structures (hexagonal close packed, face-centered cubic, or body-centered cubic structures). Relatively few studies exist on the preparation of monodisperse nonspherical colloids. In general, direct synthetic methods are restricted to particles with simple shapes such as rods, spheroids, or plates [68]. An alternative route for the preparation of uniform particles with a more complex structure might consist of the formation of discrete uniform aggregates of self-organized spherical particles. The use of colloidal clusters with a given number of particles, with controlled shape and dimension, could lead to colloidal crystals with unusual symmetries [69]. 

Next, it will be helpful to anticipate a description of experimental procedures and consider the magnitude of measured diffusion coefficients. The self-diffusion coefficients for ordinary liquids with small molecules are of the order of magnitude 10 9 m2 s for colloidal substances, they are typically of the order 10"11 m2 s l. In the next section, we see that for spherical particles the diffusion coefficient is inversely proportional to the radius of the sphere. Therefore, every increase by a factor of 10 in size decreases the diffusion coefficient by the same factor. Qualitatively, this same inverse relationship applies to nonspherical particles as well. Once again, we see that diffusion decreases in importance with increasing particle size, precisely those conditions for which sedimentation increases in importance. For larger particles, for which D is very small, the diffusion coefficient also becomes harder to measure. For... 

Hunter, R. J., Zeta Potentials in Colloid Science Principles and Applications, Academic Press, London, 1981. (Advanced level. The focus of this book is on the role of electrical double layers and zeta potential on electrophoresis and electroviscous effects. This volume presents some details on electrical double layers around nonspherical particles not discussed in the present book.)... 

What are the factors that are relevant for extending the theories of coagulation presented here to (a) polydisperse colloids, (b) nonspherical colloids, and (c) conditions for which fluid flow is important ... 

The objectives of this chapter are the following i) to briefly discuss a number of methods that have been demonstrated for the facile synthesis of spherical and nonspherical colloids with well-controlled sizes, shapes, and properties ii) to address experimental issues related to the self-assembly of spherical colloids into well-defined aggregates Hi) to demonstrate the potential of spherical colloids in producing three-dimensionally periodic lattices and iv) to assess a number of intriguing applications associated with periodic arrays of spherical colloids. 

According to the Einstein theory, the intrinsic viscosity of a spherical particle suspension is 2.5. However, for a colloidal suspension of nonspherical particles, [r ] > 2.5. Jeffery [112] obtained the viscosity of an ellipsoidal particle suspension under shear. Incorporating Jeffery s results of velocity fields around the particle, Simha [113] obtained expressions for two explicit limiting cases of ellipsoids. Kuhn and Kuhn [114] also obtained an expression for intrinsic viscosity for the full range of particle aspect ratio (p) by taking an approach similar to Simha s method. 

Lu, Y, Yin, Y, and Xia, Y, Fabrication of three-dimensional photonic crystals with nonspherical colloids as the building blocks, Adv. Mater., 13, 415, 2001. 

However, despite this progress, it is highly recommended that both PCS and LD be used simultaneously. It should be kept in mind that both methods are not measuring particle sizes. Rather, they detect light-scattering effects that are used to calculate particle size. For example, uncertainties may result from nonspherical particle shapes and from the assumption of certain parameters that are used to calculate the particle size. Platelet structures commonly occur during lipid crystallization [57] and have also been observed for SLN [11,40,58], The presence of several populations and other colloidal structures adds further difficulties. 

The application of FFF techniques to assorted silica samples including colloidal microspheres (Ludox, Monospher, and Nyacol), fumed silica, and chromatographic silica (see Table II) are discussed in this section. In several cases, different FFF systems are applied to the same samples, and the results are compared. The acquisition of size distribution data is emphasized, but the possibilities for measuring densities and porosities for spherical particles and structural factors for nonspherical silica are also discussed (and in one case demonstrated). 

As discussed in Section 5.4, the colloidal spheres can be stretched to form nonspherical colloids such as ellipsoidal colloids by Ar" laser irradiation. The array of the ellipsoidal colloids is obtained by exposing the colloidal sphere array to a polarized Ar laser beam for 10 min. The films with elliptical pores can be obtained from the array of the ellipsoidal colloids after the same annealing treatment. Figure 5.25 shows the SEM images of the array of the ellipsoidal colloids and the corresponding porous film formed from the structure inversion. The average axial ratio of the colloids and pores are 1.46 and 1.25. The smaller axial ratio for the pores could be attributed to the stress relaxation occurring in the structure inversion process. 

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