Rheological Techniques to Assess Sedimentation and Syneresis

As mentioned in Chapter 9, sedimentation is prevented by the addition of thickeners that form a three-dimensional elastic network in the continuous phase [3]. 

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 sedimentation is completely eliminated. 

The shear stress, a, exerted by a particle (force/area) can be simply calculated. 

for a 10 pm radius particle with a density difference Ap of 0.2 g cm stress is equal to, 

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. 

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