Colloidal Interactions in Aerosols

Some practical results of that strong interaction include household fuzz balls, where dust particles flocculate and form fractal-like structures (homogeneous flocculation Fig. 13.2fl) and the adhesion of aerosol particles to extended surfaces (heterogeneous flocculation. Fig. 13.2h). 

Such attractive interactions can be particularly important in situations where the presence of even a few extraneous particles on a surface can be highly detrimental, as in the production of microchips for the electronics industry (Fig. 13.3). The presence of a single dust particle on the surface of a silicon wafer before coating with the photoresist resin that will be used to engrave the final circuit will, in aU probability, result in a defective product in that area. When one considers that modern chips may have circuit line spacings of less than 10 cm, a particle of that diameter or even smaller will represent a veritable monkey wrench in the works. For that reason, extreme measures must be taken to ensure that aerosol particles are absent (to the extent technologically possible) in production areas. 

FIGURE 13.3. The absence of aerosol particles (dust or liquid) is especially important in the microelectronics industry. The presence of dust or liquid contaminants on the surface of virgin semiconductor (a) will lead to coating defects in the preparation of the microdrcuits (Z ) and defects in the final product (c). 

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Study of the dynamics of fluid flow is concerned with the forces acting on the bodies in the fluid. In the earher chapters on soUd dispersions, emulsions, and foams, fluid dynamics was largely ignored in favor of the true colloidal interactions. In aerosols, the nature of the continuous medium makes the subject of fluid dynamics much more important to the understanding of the system, so that the following discussion will introduce a few basic relationships that can be important in the study of aerosols. 

Heretofore the milieu on or in whieh the disinfectant has been required to act has been either solid or liquid now antibacterial aetion in the gas or vapour phase or in the form of aerosol (colloidal) interaction must be considered, and this presents the problem of determining the viable airborne population. 

For practical and fundamental reasons, there was a need to learn about the interactions of bodies much larger than the atoms and small molecules in gases. What interested people were systems we now call mesoscopic, with particles whose finite size Wilhelm Ostwald famously termed "the neglected dimension" 100-nm to 1 ()()-//m colloids suspended in solutions, submicrometer aerosols sprayed into air, surfaces and interfaces between condensed phases, films of nanometer to millimeter thickness. What to do ... 

In a first analysis, we can identify at least four basic differences between aerosols and other colloids related to the dispersion medium (1) buoyancy effects, (2) the effects of movement of the dispersing medium, (3) particle mobility in undisturbed conditions (i.e., free fall), and (4) modification of interactions by the intervening medium. In emulsions, foams, and sols we have seen that buoyancy can be important in determining the stability of a system (i.e., matching the densities of dispersed and continuous phases can retard creaming or sedimentation). In aerosols, where the density of the continuous phase will always be significantly less than that of the dispersed particle, such effects are practically nonexistent—the colloid is essentially left to its own devices the usual interactions found for all colloids, the constant ... 

For more practical purposes, therefore, one should take recourse to metal particles as produced by other means, in particular on supports or in matrices. The advantage is the availability of macroscopic amounts of sample the disadvantage is that interaction with the supporting medium must be assessed. A great variety of synthetic methods exists, of which we can mention only a few. Metal clusters can be produced by aerosol techniques, by vapor deposition, by condensation in rare-gas matrices, by chemical reactions in various supports, e.g. zeolites, SiOi, AI2O3, or polymer matrices. Many different metal-nonmetal composites, such as the ceramic metals (cermets) have been obtained with metal particles with sizes varying from nanometers upward. In alternative approaches, metal particles are stabilized by chemical coordination with ligand molecules, as in metal colloids and metal cluster compounds. 

Fig. 2 shows the different pathways in which chemical elements contained in rocks are released to the different environmental compartments. Five main processes are responsible for their dispersion into the different ecosystems (1) Weathering, either directly by rain water on rock outcrops, by soil percolation water or by root exsu-dates, which interact with rock fragments, contained in the soil cover (2) Down hill mechanical transport of weathered rock particles, such as creep and erosion and subsequent sedimentation as till material or alluvial river and lake sediments (3) Transport in dissolved or low size colloidal form by surface and groundwater (4) Terrestrial and aquatic plants growing in undisturbed natural situations will take up whatever chemical elements they need and which are available in the surface and shallow groundwater. Trace elements taken up from the soil will accumulate in the leaves and will possibly enrich the soil by litterfall (5) Diffuse atmospheric input by aerosols and rain rock particles from volcanic eruptions, desertic areas (Chester et al., 1996), seaspray and their reaction with rain water. A considerable part of this can be anthropogenic. 

Pailthorpe and Russel (1982) and Aninachalam et at. (1998) compared this approximation with the exact solution by Langbein for colloidal polystyrene spheres in an aqueous solution and for aerosol molecule clusters of tetrachloromethane (CCLi), respectively. In both cases the approximate approach underestimates the van-der-Waals interaction energy by approx. 10 to 20 %. Pailthorpe and Russel (1982) conclude that this deviation is comparable with the uncertainty due to inaccurate dielectric values. 

Mists and fogs are colloidal dispersions of a liquid in a gas. They may therefore be regarded as being the inverse of foams. The interactions controlling their stability, however, are not generally the same as those involved in foam stabilization, because most mists and fogs do not possess the thin lamellar stabilizing films encountered in foams. In fact, the stability of liquid aerosols is usually more dependent on fluid dynamics than on colloidal factors, as illustrated below. 

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