Gravity-induced flocculation

In this paper we propose a simple procedure whereby one can estimate the overall stability of a given colloidal system undergoing simultaneous creaming and flocculation. Since case (iii) has not been solved yet, we will focus our attention only on case (ii). As an example, we use this procedure to compare the net rates of particle loss due to creaming and gravity-induced flocculation when electrostatic repulsion is negligible. 

For a polydisperse system of particles undergoing both creaming and gravity-induced flocculation, the time evolution of the particle size distribution, and hence the time rate of change of the total particle concentration, can only be determined from the solution of the governing population balance equations (10,11). For the special case of two different sized particles, this system of balance equations reduces to ... 

The rest of this paper will be devoted to studying the relative rates of creaming and gravity-induced flocculation, that is... 

Ogr the gravity-induced flocculation capture efficiency which, for... 

Figure 3 shows the effect of increasing particle size on the relative rates of creaming and gravity-induced flocculation. As the particle size increases, the rate of flocculation increases faster than the creaming rate of the particles. This is a result which, at first, one wouldn t expect. According to Equations (4), (10), and (11), the total rate of particle loss due to creaming R is... 

He have presented a simple procedure whereby one can estimate the stability of a colloidal system undergoing simultaneous creaming and gravity-induced flocculation. This procedure is by no means restricted to only this case. One can easily take into account other particle loss mechanisms, such as shear-induced flocculation or Brownian flocculation. What is required in these cases are the appropriate particle/particle collision kernels, which can be computed by solving the governing convective-diffusion equation. 

The main mechanisms of instability that are involved in leading to complete phase separation of emulsions are creaming [64], flocculation [65,66], coalescence [67], and Ostwald ripening [68,69]. However, nano-emulsions do not cream (or sediment) because the Brownian motion is larger than the small creaming rate induced by gravity. Practically, the creaming of droplets smaller than 1 im is stopped by their faster diffusion rate. 

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