Stability Against Claying or Caking

Once a dispersion that is stable in the colloid sense has been prepared, the next task is to eliminate claying or caking. This is the consequence of settling of the 

The sedimentation velocity Vq of a very dilute suspension of rigid non-interacting particles vith radius a can be determined by equating the gravitational force with the opposing hydrodynamic force as given by Stokes law, i.e. 

The above treatment using Stokes law applied only to very dilute suspensions (volume fraction j 0.01). For more concentrated suspensions, the particles no longer sediment independent of each other and one has to take into account both the hydrodynamic interaction between the particles (which applies for moderately concentrated suspensions) and other higher order interactions at relatively high volume fractions. A theoretical relationship between the sedimentation velocity v of non-flocculated suspensions and particle volume fraction has been derived by Maude and Whitmore [87] and by Batchelor [88]. Such theories apply to relatively low volume fractions ( 0.1) and they show that the sedimentation velocity i at a volume fraction rj) is related to that at infinite dilution (the Stokes velocity) by an equation of the form 

Reed and Anderson [89] developed a virial expansion technique to describe the set- 

Good agreement between experimental settling rates and those calculated using Eq. (14.39) was obtained up to (3 = 0.4. 

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