Creaming or Sedimentation Rates

Creaming or Sedimentation Rates 10.6.1.1 Very Dilute Emulsions [cp 0.01) 

In this case, the rate can be calculated using Stokes law, which balances the hydrodynamic force with gravity force  

Here one has to take into account the hydrodynamic interaction between the droplets, which reduces the Stokes velocity to a i given by the following expression. 

for emulsions that are deformable, can be much larger than 0.74. 

Several procedures that may be applied to reduce or eliminate creaming or sedimentation are discussed below. 

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Figure 21.1 Variation of creaming or sedimentation rate with residual viscosity.

As discussed in the suspensions section, the driving force for creaming (whereby the droplets have a density lower than the medium) or sedimentation is gravity. When the gravity force exceeds the Brownian diffusion, creaming or sedimentation will occur. With macroemulsions (droplets > 1 pm), the creaming or sedimentation rate is very fast and it may be completed in a matter of hours or days. 

Equation (14.18) only applies for an infinitely dilute emulsion. For a concentrated emulsion, the creaming or sedimentation rate is v reduced with increasing volume fraction of the emulsion. This can be empirically expressed as... 

Case (c) is that for a polydisperse (practical) emulsions, in which case the droplets will cream or sediment at various rates. In this last case a concentration gradient build-up occurs, with the larger droplets staying at the top of the cream layer or the bottom of the sediment ... 

The rate of creaming or sedimentation becomes a complex function of 4>, as illustrated in Figure 10.24, which also shows the change of relative viscosity with [Pg.189]

The above-described thickeners satisfy the criteria for obtaining very high viscosities at low stresses or shear rates. This can be illustrated from plots of shear stress a and viscosity tj versus shear rate y (or shear stress), as shown in Figure 10.25. These systems are described as pseudoplastic or shear thinning. The low shear (residual or zero shear rate) viscosity tj(0) can reach several thousand Pa s, and such high values prevent creaming or sedimentation [24, 25]. 

The above-described behaviour is obtained above a critical polymer concentration (C ), which can be located from plots of log tj versus log C, as is illustrated in Figure 10.26. Below C, the log //-log C curve has a slope in the region of 1, whereas above C the slope of the line exceeds 3. In most cases a good correlation between the rate of creaming or sedimentation and //(O) is obtained. 

For smaller particles, smaller stresses are exerted. Thus, in order to predict sedimentation it is necessary to measure the viscosity at very low stresses (or shear rates). These measurements can be carried out using a constant stress rheometer (Carrimed, Bohlin, Rheometrics, Haake or Physica). Usually, a good correlation is obtained between the rate of creaming or sedimentation, v, and the residual viscosity rj 0), as will be described in Chapter 21. Above a certain value of ri(0), v becomes equal to 0. Clearly, in order to minimize sedimentation it is necessary to increase rj 0) an acceptable level for the high shear viscosity must be achieved, depending on the application. In some cases, a high rj[0) may be accompanied by a high rj (which may not be acceptable for apphcation, for example if spontaneous dispersion on dilution is required). If this is the case, the formulation chemist should seek an alternative thickener. 

Sedimentation or creaming is prevented by the addition of thickeners that form a three-dimensional elastic network in the continuous phase. If the viscosity of the elastic network, at shear stresses (or shear rates) comparable to those exerted by the particles or droplets, exceeds a certain value, then creaming or sedimentation is completely eliminated. 

Thus, to predict creaming or sedimentation, it is necessary to measure the viscosity at very low stresses (or shear rates), and these measurements can be carried out using a constant stress rheometer (e.g., Carrimed, Bohlin, Rheometrics, Haake or Physica). 

Creaming or sedimentation of emulsions with dro plet sizes above 1 pm eauses some experimental diffr culty because of the ehange in the total amount of spins in the NMR-aetive volume of the sample tube diuing the experiment. This can be accounted for by extra reference measurements with no gradient applied before and after each NMR scan at a particular value of 5. In addition, such reference measurements may provide information on the creaming rate which is a useful characteristic of emulsions. 

Stability of a macroemulsion is an important factor as this determines its extent of usability for particle preparation or various other applications. Instability is basically coalescence of the dispersed phase droplets or Ostwald ripening (growth of large droplets at the expense of much smaller ones). When this process goes on, the emulsion eventually breaks into two layers. Other processes related to stability but considered less important [3] are (a) creaming or sedimentation, the rate of which is dependent on the difference in density between the continuous and dispersed phases, droplet size, viscosity of the continuous phase and interdroplet interaction and (b) flocculation, dependent on colloidal interactions between the droplets [8, 12]. Several factors determine the stability of macroemulsions these are discussed here in brief. This discussion is largely derived from Rosen [3] and some subsequent investigations [e.g. 6, 7, 13-15]. 

The rate of creaming or sedimentation becomes a complex function of as is illustrated in Figure 6.25, which also shows the change of relative viscosity 7, with As seen in figure, v decreases with increasing and ultimately approaches zero when (j> exceeds a critical value (. ), which is the so-called maximum pack-... 

These systems are described as pseudo-plastic or shear thinning. The low shear (residual or zero shear rate) viscosity /(o) can reach several thousand Pa s and such high values prevent creaming or sedimentation. 

In most cases, good correlation between the rate of creaming or sedimentation and //(o) is obtained. 

Measurement of the rate by direct observation of emulsion separation using graduated cylinders that are placed at constant temperature. This method allows one to obtain the rate as well as the equUibrium cream or sediment volume. 

Centrifugation may be applied to accelerate the rate of creaming or sedimentation, but one should be careful in the amount of g force that may be apphed (g should not exceed the critical g force that causes deformation of the emulsion droplets and oil separation). 

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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