Colloidal systems particle shape

The viscosity of colloidal dispersions is affected by the shape of the dispersed phases. Sphero-colloids form dispersions of relatively low viscosity, while systems containing linear particles are generally more viscous. The relationship of particle shape and viscosity reflects the degree of solvation of the particles. In... 

In this group of disperse systems we will focus on particles, which could be solid, liquid or gaseous, dispersed in a liquid medium. The particle size may be a few nanometres up to a few micrometres. Above this size the chemical nature of the particles rapidly becomes unimportant and the hydrodynamic interactions, particle shape and geometry dominate the flow. This is also our starting point for particles within the colloidal domain although we will see that interparticle forces are of great importance. 

This latter case is the same result as Einstein calculated for the situation where slip occurred at the rigid particle-liquid interface. Cox15 has extended the analysis of drop shape and orientation to a wider range of conditions, but for typical colloidal systems the deformation remains small at shear rates normally accessible in the rheometer. The data shown in Figure 3.11 was calculated from Cox s analysis. His results have been confirmed by Torza et al.16 with optical measurements. The ratio of the viscous to interfacial tension forces, Rf, was given as ... 

Monodisperse spheres are not only uniquely easy to characterize, but also very rarely encountered. Polymerization under carefully controlled conditions allows the preparation of the polystyrene latex shown in Figure 1.8. Latexes of this sort are used as standards for the size calibration of optical and electron micrographs (also see Section 1.5a.3). However, in the majority of colloidal systems, the particles are neither spherical nor monodisperse, but it is often useful to define convenient effective linear dimensions that are representative of the sizes and shapes of the particles. There are many ways of doing this, and whether they are appropriate or not depends on the use of such dimensions in practice. There are excellent books devoted to this topic (see, for example, Allen 1990) and, therefore, we consider only a few examples here for the purpose of illustration. 

We have noted previously that Rg is related to the geometrical dimensions of a body through expressions that are specific for the particle shape. Table 5.4 lists some of these relationships for shapes pertinent to colloidal systems. For a selected shape, one of the tabulated relationships can be used to replace Rg in Equation (74). What results is an expression that interprets z in terms of actual particle dimensions for the geometry chosen. 

The factors which contribute most to the behaviour of colloidal systems are the dimensions, the shape and the properties of the surfaces of the particles, but also the medium in which they are dispersed is of influence. The large to extremely large ratios between the surfaces and the volumes of the particles are of importance for all of these properties. 

Particle asymmetry is a factor of considerable importance in determining the overall properties (especially those of a mechanical nature) of colloidal systems. Roughly speaking, colloidal particles can be classified according to shape as corpuscular, laminar or linear (see, for example, the electron micrographs in Figure 3.2). The exact shape may be complex but, to a first approximation, the particles can often be treated theoretically in terms of models which have relatively simple shapes (Figure 1.1). 

As we shall see, the intensity, polarisation and angular distribution of the light scattered from a colloidal system depend on the size and shape of the scattering particles, the interactions between them, and the difference between the refractive indices of the particles and the dispersion medium. Light-scattering measurements are, therefore, of great value for estimating particle size, shape and interactions, and have found wide application in the study of colloidal dispersions, association colloids, and solutions of natural and synthetic macro-molecules. 

The properties of the colloidal systems are considerably influenced by factors such as polydispersity, and particle s size and shape. However, from the theoretical and experimental point of view, the methods of determining... 

Energy can also be stored in other ways on a microscopic scale, e.g., by electrical charges being forced near each other in colloidal systems and by emulsion drops being distorted from the spherical shape. In this case, the surface tension gives them stabilizing surfactant layers on dispersed particles being pressed into each other. 

B. A. Keiser s contribution to this book (the introduction to the section Preparation and Stability of Sols ) constitutes an excellent introduction to silica nucleation, polymerization, and growth in both aqueous and alcoholic systems for the preparation of silica sols. Yoshida s chapter (Chapter 2) focuses on industrial development in the preparation of monodisperse sols from sodium silicate and predicts further progress in the development of silica sols that have shapes other than spherical, such as elongated, fibrous, and platelet. Colloidal silica particles with these shapes show novel properties and open the possibility of new industrial applications. 

Tanori J and Pileni M P 1997 Control of the shape of copper metallic particles by using a colloidal system as template Langmuir Z 639... 

---