Physical stability of suspensions and emulsions

The main driving force for separation of suspensions or emulsions is gravity. Most suspensions or emulsions have particle or droplet size ranges whereby the Brownian diffusion kT (where k is the Boltzmann constant and T is the absolute temperature) of the particles or droplets is insufficient to overcome the gravity force (that is given by the mass of each particle x acceleration due to gravity g x height of the container I), i.e. 

R is the particle or droplet radius, Ap is the density difference between particles or droplets and the medium. 

To prevent sedimentation or creaming, one needs to add suspending agents in the continuous phase. Such agents are usually high molecular weight polymers (such as hydroxyethyl cellulose or xanthan gum), usually referred to as thickeners. 

These thickeners produce very high viscosities at low shear rates or shear stresses and hence they overcome the stresses exerted by the sedimenting or creaming particles of droplets. Generally, these high molecular weight polymers produce non-Newtonian flow with a yield stress that prevents separation on storage. 

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