Ostwald flocculation assessment

Two main procedures can be applied for the characterisation of suspensions and assessment of their stability (such as flocculation). The first method depends on the measurement of particle size distribution and the rate of flocculation and/or Ostwald ripening after dilution of the suspension with the dispersion medium, while the second procedure depends on measurement of the state of suspension without dilution, using rheological techniques. As both methods are described in detail in Chapters 19 and 20, only a summary will be provided here. 

In this chapter, a summary of the methods that can be applied to assess the structure of the solid/Uquid interface will first be provided, followed by details of assessing sedimentation, flocculation, and Ostwald ripening. In the latter cases (flocculation and Oswald ripening), information is needed on the particle size distribution, and several techniques are available to obtain this from diluted systems. It is essential to dilute the concentrated suspension with its own dispersion medium in order not to affect the state of the dispersion during examination. The dispersion medium can be obtained by centrifuging the suspension, when the supernatant liquid will be produced at the top of the centrifuge tube in the case... 

Assessment of Flocculation and Ostwald Ripening (Crystal Growth)... 

The assessment of flocculation and Ostwald ripening of a suspension requires measurement of the particle size and shape distribution as a function of time. Several techniques may be apphed for this purpose, and these are summarised below [6]. 

The above measurements should be supplemented by particle size distribution measurements of the diluted dispersion (ensuring that no floes are present after dilution) to assess the extent of Ostwald ripening. Another compHcation may arise from the nature of the flocculation which, if it occurs in an irregular fashion (producing strong and tight floes), may cause i (0) to be increased while cr may show some decrease, and this wiU complicate the analysis of the results. Yet, in spite of these complications, constant stress measurements may provide valuable information on the state of the dispersion on storage. 

The above trend depends on the extent of flocculation relative to Ostwald ripening. Therefore, following ap and with storage time requires knowledge of Ostwald ripening and/or coalescence. Only in the absence of this latter breakdown process can one use rheological measurements as a guide of assessment of flocculation. 

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