Dispersion of Colloidal Suspensions

This section addresses the mechanisms of agglomerate breakup and the relevant dispersion techniques. It reviews experimental reports on the dispersion of pyrogenic powders with focus on empirical scaling laws of dispersion. Finally, the section discusses the applicability of various characterisation techniques to monitor the dispersion of colloidal suspensions. 

The dispersion of agglomerates requires that they experience a sufficiently high mechanical stress Tdisp that eventually exceeds the agglomerate strength agg- The ratio of the two quantities, the fragmentation number Fa, is thus a measure on the effectiveness of the dispersion process (Rwei et al. 1990 Baldyga et al. 2008)  

This equation holds true for arbitrarily shaped primary particles, which may be compact individuals (as considered by Rumpf) or aggregates of even finer particles (Baldyga et al. 2008). The equation can be further refined by considering the interdependence of porosity e gg and coordination number Nc- Rumpf proposed a 

Note that for fractal agglomerates, the porosity Cagg is function of the 

Additionally, the type of deagglomeration may depend on the type of stress that is exerted to the agglomerate. Deagglomeration in colloidal suspensions mainly results from the interaction with the continuous phase, i.e. from viscous forces and pressure fluctuations. The stress on the agglomerates is, thus, closely related to the flow field  

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J.A. Lewis, G. Kirby, J.H.-W. Cheung, and A.A. Jeknavorian, Controlled dispersion of colloidal suspension by comb polymers, US Patent 7 053125, assigned to The Board of Trustees of the University of Illinois (Urbana, IL) W.R. Grace Co.-Conn. (Columbia, MD), May 30, 2006. 

In summary, ultrasonication facilitates the dispersion of colloidal suspensions at relatively high stress intensities. It relies on the presence of cavitation and on a deep penetration of the sound field into the suspension. The latter is guaranteed by low suspension viscosities (e.g. as for aqueous media with low solid content). A particular problem is the abrasion of the sonotrode tip, which depends on the solid phase, the particle size and shape, the sound intensity, as well as on the abrasion history of the tip. Abrasion particles from sonotrodes are coarse particles in the micrometre range and may spoil the quality of colloidal suspensions. 

The defined dispersion of particle agglomerates and aggregates is an indispensable step when studying the macroscopic properties of colloidal suspensions or measuring their state of dispersion. In that regard, there is a vast number of publications that provide experimental data on the dispersion of colloidal suspensions. However, only a few of them systematically investigate the main factors of dispersion processes as well as to which extent a scaling and comparison of dispersion processes is possible. Based on these publications, one can identify a set of parameters that determine the effectiveness of dispersion ... 

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