Aerosol flocculation

In the derivations of the above equations, image forces (which are likely to be small compared to the Coulomb forces) were neglected and every collision was assumed to result in adhesion. Although some have questioned the validity of the latter assumption, there is no evidence to indicate that this is not, at least nominally, the case when dealing with particles smaller than 10 microns colliding at low velocities. While this subject of adhesion has never been investigated systematically, the bulk of all the data on aerosol flocculation and deposition would imply that for these conditions adhesion... 

Fig. XIV-9. Effects of electrolyte on the rate of flocculation of Aerosol MA-stabilized emulsions. (From Ref. 35.)...

An interesting example of a large specific surface which is wholly external in nature is provided by a dispersed aerosol composed of fine particles free of cracks and fissures. As soon as the aerosol settles out, of course, its particles come into contact with one another and form aggregates but if the particles are spherical, more particularly if the material is hard, the particle-to-particle contacts will be very small in area the interparticulate junctions will then be so weak that many of them will become broken apart during mechanical handling, or be prized open by the film of adsorbate during an adsorption experiment. In favourable cases the flocculated specimen may have so open a structure that it behaves, as far as its adsorptive properties are concerned, as a completely non-porous material. Solids of this kind are of importance because of their relevance to standard adsorption isotherms (cf. Section 2.12) which play a fundamental role in procedures for the evaluation of specific surface area and pore size distribution by adsorption methods. 

In addition to the deposition mechanisms themselves, methods for preliminary conditioning of aerosols may be used to increase the effectiveness of the deposition mechanisms subsequently apphed. One such conditioning method consists of imposing on the gas nigh-intensity acoustic vibrations to cause collisions and flocculation of the aerosol particles, producing large particles that can be separated by simple inertial devices such as cyclones. This process, termed sonic (or acoustic) agglomeration, has attained only hmited commercial acceptance. 

A cloud is essentially an aerosol with mobile boundaries. The stability of a cloud involves two distinct aspects. Because of Coulomb, image, and diffusion forces, particles in a given volume will flocculate. This will result... 

Having considered the factors that govern the extent or rate of flocculation in an aerosol, it is also important to consider the effects of flocculation on the properties of an aerosol. Consider a large sphere of diameter Dpl, with its surface covered with small particles of diameter Dpi. If the adhering particles are no more than one layer thick, it can be shown as a good first approximation that... 

Ambipolar charges can give increased deposition but such deposition depends for the most part on image effects. In this sense the rate of deposition will be comparable with the rate of flocculation of ambipolar aerosols. Improved deposition rates by such means are likely to be small. 

Ranucci, J. A., Dixit, S., Bray, R. N., Goldman, D. Controlled flocculation in metered-dose aerosol suspensions. Pharm Tech 14(4) 68, 70-72, 74 (1990). 

Flocculation of aerosols by high frequency sound. (Processed.) Given at dedication of U. S. Bur. Mines Exp. Sta., Salt Lake City, Utah. 

A final assumption made in the derivation of Stokes law was that the particles of interest were spheres. In many cases this is not true. Particles may have irregular shapes, depending on how they were formed and the amount of agglomeration which may have taken place. Liquid aerosols are always spherical, so that for liquid aerosols the assumption of sphericity holds. For isometric particles this assumption can also be used with little error. For long chains of particles or flocculated particles, large deviations from Stokes law are possible. 

Effect of Demulsifier Mixture. In previous studies (27) Duo-meen C, which was effective in causing flocculation of the water droplets, was not very effective in breaking the interfacial film formed between the water droplets, which inhibits coalescence. (Duomeen C is a mixture of many types of surfactants the general classification is a fatty acid ester nitrogen derivative.) However, Duomeen C in combination with docusate sodium (Aerosol OT), a hydrophilic surfactant, was much more effective in causing water separation compared to the individual chemicals. This effect is shown in Figure 16 for a 6 vol% water-in-oil (Leduc crude) emulsion in which both the UVP signal (20 min after chemical addition) and the volume... 

Controlled flocculation In metered-dose aerosol suspensions... 

Ranucci, J.A. Dixit, S. Bray, R., Jr. Goldman, D. Application of controlled flocculation to metered dose aerosols suspensions. Pharm. Res. 1987, 4 (2 suppl.), S-25. 

The van der Waals (VDW) attractive forces are the principal forces between dry, noncharged spherical aerosol particles [262] and may reduce stability and cause flocculation of suspended particles. The VDW forces arise from the attractive forces between permanent dipoles (Keesom forces), induced dipoles (London forces), and dipole-induced dipoles (Debye forces). For nonpolar or slightly polar compounds, the force of attraction between two particles with diameter d separated by a distance h (where h < d) is ... 

Studies of the atmospheric input of chemicals to the open ocean have also been increasing lately. For many substances a relatively small fraction of the material delivered to estuaries and the coastal zone by rivers and streams makes its way through the near shore environment to open ocean regions. Most of this material is lost via flocculation and sedimentation to the sediments as it passes from the freshwater environment to open sea water. Since aerosol particles in the size range of a few micrometers or less have atmospheric residence times of one to several days, depending upon their size distribution and local precipitation patterns, and most substances of interest in the gas phase have similar or even longer atmospheric residence times, there is ample opportunity... 

Friedlander (11) has examined the effects of flocculation by Brownian diffusion and removal by sedimentation on the shape of the particle size distribution function as expressed by Equation 9. The examination is conceptual the predictions are consistent with some observations of atmospheric aerosols. For small particles, where flocculation by Brownian diffusion is predominant, p is predicted to be 2.5. For larger particles, where removal by settling occurs, p is predicted to be 4.75. Hunt (JO) has extended this analysis to include flocculation by fluid shear (velocity gradients) and by differential settling. For these processes, p is predicted to be 4 for flocculation by fluid shear and 4.5 when flocculation by differential settling predominates. These theoretical predictions are consistent with the range of values for p observed in aquatic systems. 

Let us consider the case of aerosol suspensions. The suspended particles must be well dispersed so as to penetrate the lung. In this sense, an ideal aerosol suspensiem docs not sediment and form aggregates. However, particles eventually do aggregate, so it is convenient to formulate them in such a way as to promote weak flocculates that can be easily redispersed by shear (e.g agitation of the aerosol ermtainer). The formulation can be adjusted mea.suring viscosity as a function of shear rate or tdiear stress, as depicted in Refs. 6 and 30. 

Uses Suspending agent, gellant, binder for household prods., pharmaceuticals, automotive prods., aerosols, paints, enamels, personal care prods. flocculant for water purification/treatment systems, industrial... 

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