Thin liquid films colloidal particle

Keywords Colloidal particles Thin liquid films Structural forces Colloidal... 

Oscillatory structural forces appear in two cases (1) in thin films of pme solvent between two smooth solid surfaces (2) in thin liquid films containing colloidal particles (including macromolecules and surfactant micelles). In the first case, the oscillatory forces are called the solvation... 

In fact, Equation 5.281 describes an interface as a two-dimensional Newtonian fluid. On the other hand, a number of non-Newtonian interfacial rheological models have been described in the literature. Tambe and Sharma modeled the hydrodynamics of thin liquid films bounded by viscoelastic interfaces, which obey a generalized Maxwell model for the interfacial stress tensor. These authors also presented a constitutive equation to describe the rheological properties of fluid interfaces containing colloidal particles. A new constitutive equation for the total stress was proposed by Horozov et al. ° and Danov et al. who applied a local approach to the interfacial dilatation of adsorption layers. 

Additional support for a nucleation and aggregation model is provided by cryo-TEM results of Bailey and Mecartney (116). By fast-freezing thin liquid films of sol, they were able to follow the evolution of particle growth in situ without artifacts introduced by drying. They concluded that particle growth occurs by addition of small, low-density particles to larger, colloidally stable seeds . A similar hypothesis was made by Iler (2) on the basis of some very early TEM work by Radczewski and Richter (117) on sols prepared by hydrolysis of SiCU. 

Definition and Classification of Foams. Colloidal species of any kind (bubbles, particles, or droplets), as they are visually defined, have at least one dimension between 1 and 1000 nm. Foams are a special kind of colloidal dispersion one in which a gas is dispersed in a continuous liquid phase. The dispersed phase is sometimes referred to as the internal (disperse) phase, and the continuous phase as the external phase. In practical occurrences of foams, the bubble sizes usually exceed the size limit given, as may the thin liquid-film thicknesses. Table II lists some simple examples of petroleum industry foam types. Solid foams, dispersions of gas in a solid, will not in general be covered in this chapter. A glossary of frequently encountered foam terms in the science and engineering of petroleum industry foams is given at the end of this volume. 

So far we have been concerned mainly with dispersions of solids in which all three dimensions of the particles of the dispersed phase are in the colloid size range. An important group of colloidal phenomena involve systems having only one dimension in this range (see Figure 1.1). This chapter deals with thin liquid films, present in isolation, as the basic components of liquid foams or as the film between two emulsion droplets in contact. Other examples of such colloidal systems are those in which the solid particles are thin plates, as in many clay systems, and solid foams. These will, however, be omitted from the present discussion. 

Oscillatory structural forces appear in thin films of pure solvent between two smooth solid surfaces and in thin liquid films containing colloidal particles including macromolecules and surfactant micelles (Israelachvili 1992). In the first case, the oscillatory forces are called the solvation forces and they are important for the short-range interactions between solid particles and dispersions. In the second case, the structural forces affect the stability of foam and emulsion films as well as the flocculation processes in various colloids. At lower particle concentrations, the structural forces degenerate into the so-called depletion attraction, which is found to destabilize various dispersions. 

Thin liquid films can be formed between two coUiding emulsion droplets or between the bubbles in foam. Formation of thin films accompanies the particle-particle and particle-wall interactions in colloids. From a mathematical viewpoint, a film is thin when its thickness is much smaller than its lateral dimension. From a physical viewpoint, a liquid film formed between two macroscopic phases is thin when the energy of interaction between the two phases across the film is not negligible. The specific forces causing the interactions in a thin liquid film are called surface forces. Repulsive surface forces stabilize thin films and dispersions, whereas attractive surface forces cause film rupture and coagulation. This section is devoted to the macroscopic (hydrostatic and thermodynamic) theory of thin films, while the molecular theory of surface forces is reviewed in Section 4.4. 

The interactions across a thin film, called the surface forces, to a great extent are determined by the surfactant adsorption at the film surfaces. From a physical viewpoint, a liquid film formed between two phases is termed thin when the interaction of the phases across the film is not negligible. Thin films appear between the bubbles in foams, between the droplets in emulsions, as well as between the particles in suspensions. Furthermore, the properties of the thin liquid film determine the stability of various colloids. 

FIGURE 2.3 A colloidal particle in a thin liquid film, either sustained by a solid substrate (left sketch, film thickness R + h) or forming a liquid bridge (right sketch, film thickness 2h). 

Lateral forces between particles floating at a fluid interface or immersed in a thin liquid film are observed and utilized in some extraction and separation flotation processes [591]. They are important in stabilizing emulsions and foams, for making porous materials from emulsions and foams [593, 594], and for preparing colloidal crystals [595, 596]. The presence of lateral capillary forces has... 

The phenomena described above have been known for a long time indeed, Newton reported on the black spots in soap films. In the past 25 years, however, these thin, liquid structures have become a subject of intensive scientific studies. One of the main reasons is that the interaction forces between colloidal particles suspended in a liquid are of the same nature as those operating in soap films. Because the film geometry is well defined (i.e., a thin, flat liquid sheet, macroscopic in lateral extension), it is an attractive experimental subject for studying these forces, in particular with optical means. 

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