Mechanisms of dispersion

K. E. J. Barrett and H. R. Thomas, Kinetics and Mechanism of Dispersion Polymerization, in Dispersion Polymerization in Organic Media, K. E. J. Barret, Ed., F. Wiley and Sons, London, 1975. 

When a certain volume of a sample is instantaneously introduced into the stream of the carrier solution, the sample is carried away by the carrier not as a compact plug but gradually mixes with it. Mixing of the sample with the flowing solution is achieved by the action of two different mechanisms of dispersion axial dispersion (parallel to the direction of the flow) due to the continuous flow of the stream, and radial dispersion (in vertical direction to the flow) due to diffusion. 

The terms arise from the mechanisms of dispersion, or band-broadening, listed in Table 19.1. 

The general procedure for dispersion involves use of sonication, which causes an unzipping mechanism of dispersion as proposed by Smalley and co-workers [51]. Typical surfactants used for this purpose are sodium dodecylsulfate (SDS) or sodium dodecylbenzene sulfonate (SDBS). The latter shows a stronger interaction as a result of the presence of the aromatic ring. It also appears that longer and more branched hydrocarbon chains interact more efficiently [52],... 

This section, discusses the mechanism of dispersion polymerization, role of components, control of particle size, and the application to new systems by tracing its history. 

The nucleation mechanism of dispersion polymerization of low molecular weight monomers in the presence of classical stabilizers was investigated in detail by several groups [2,6,7]. It was, for example, reported that the particle size increased with increasing amount of water in the continuous phase (water/eth-anol), the final latex radius in their dispersion system being inversely proportional to the solubility parameter of the medium [8]. In contrast, Paine et al.[7] reported that the final particle diameter showed a maximum when Hansen polarity and the hydrogen-bonding term in the solubility parameter were close to those of steric stabilizer. 

The number of PPE particles dispersed in the SAN matrix, i.e., the potential nucleation density for foam cells, is a result of the competing mechanisms of dispersion and coalescence. Dispersion dominates only at rather small contents of the dispersed blend phase, up to the so-called percolation limit which again depends on the particular blend system. The size of the dispersed phase is controlled by the processing history and physical characteristics of the two blend phases, such as the viscosity ratio, the interfacial tension and the viscoelastic behavior. While a continuous increase in nucleation density with PPE content is found below the percolation limit, the phase size and in turn the nucleation density reduces again at elevated contents. Experimentally, it was found that the particle size of immiscible blends, d, follows the relation d --6 I Cdispersed phase and C is a material constant depending on the blend system. Subsequently, the theoretical nucleation density, N , is given by... 

The mechanism of dispersion nonadditivity was proposed over 50 years ago by Axilrod and Teller [76] and independently by Muto [77]. It is referred to as the correlation of three instantaneous dipoles. To better appreciate the behavior of this term, let us consider the same two extreme configurations of a trimer as for the TE nonadditivity described earlier. In the equilateral triangle the three monomers cooperate in correlating with each other i.e. when a third monomer gets close, it sees the other two conveniently pre-correlated. In contrast, for the collinear approach of a third monomer this pre-correlation takes place in the wrong direction. Since pair dispersion interaction is attractive, the nonadditivity is repulsive for the equilateral trimer and attractive in the collinear form. 

For inks which contain pigments, the most common problem is aggregation of the pigment particles due to the inherent instability of most dispersion systems. Since most modern inkjet inks for graphic applications contain dispersed pigments, the stabilization mechanisms of dispersions will be briefly discussed below. 

Mast, in a pioneering 1972 paper, reported visual observations of foam flow in etched glass micromodels (37 ) His observations showed that some of the conflicting claims about the properties of foam flow in porous media were probably due simply to the dominance of different mechanisms under the various conditions employed by the separate researchers (37). Mast observed most of the various mechanisms of dispersion formation, flow, and breakdown that are now believed to control the sweep control properties of surfactant-based mobility control (37,39-41). 

The development of improved methods of surfactant design required progress in several other areas (1) understanding the mechanisms of dispersion flow in porous media, to determine which physical properties should be measured, and how their values would affect sweep control (2) measurements of these properties that are valid at the conditions under which the surfactants will be used and (3) understanding of how the values of these parameters depend on phase behavior, molecular structure, and other thermodynamic variables. 

It should be noted that the field tests were made with only one type of surfactant, and without benefit of many recent research advances in such areas as high-pressure phase behavior and surfactant design, mechanisms of dispersion formation and disappearance, and mechanisms of dispersion flow through porous media. Furthermore, the design and successful performance of field tests pose many technological challenges in addition to those encountered in the prerequisite experimental and theoretical research. 

More field tests will be needed, especially to incorporate research advances in such areas as materials design phase behavior and dispersion morphology mechanisms of dispersion formation, flow, and breakdown and simulation of dispersion-based sweep control. 

The next stage in such complex approach must be an investigation of polymer foams in the physics and mechanics of disperse systems... 

An alternative dry powder aerosol device is illustrated in Fig. 9.46 and the mechanism of dispersion of powdered dmg in a Ventodisk or Becodisk system is shown in Fig. 9.47... 

The key physical mechanisms of dispersion on this scale are the differences in mean wind speed and velocity (Uc), Uh within and above the canopy. In porous canopies, the mean wind speed normalised on Uh, i.e. (Uc/Uh) is greater than the turbulence intensities ctv/Uh, o-w/Uh 1/10, so that the cloud/plume is advected by the main wind within as well as above the canopy. In addition the topological and wake dispersion processes (described in Section 2.4.2) are as significant as turbulent eddying for dispersing matter both horizontally and vertically within the canopy. 

Dispersibility of powders in the airflow is defined by the balance of forces generated by the mechanical stresses within the dispersion device and the interparticulate forces required to separate the primary particles simultaneously (24). The mechanism of dispersion is very complex and may involve dispersion by acceleration and by shear flow, as well as by impaction or other mechanical forces. 

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