Concentrated suspensions depletion flocculation

On standing, concentrated suspensions reach various states (structures) that are determined by (1) Magnitude and balance of the various interaction forces, electrostatic repulsion, steric repulsion and van der Waals attraction. (2) Particle size and shape distribution. (3) Density difference between disperse phase and medium, which determines the sedimentation characteristics. (4) Conditions and prehistory of the suspension, e.g. agitation, which determines the structure of the floes formed (chain aggregates, compact clusters, etc.). (5) Presence of additives, e.g. high molecular weight polymers that may cause bridging or depletion flocculation. 

Another method of reducing sedimentation is to employ the principle of depletion flocculation (described in Chapter 7). The addition of free (non-adsorbing) polymer can induce weak flocculation of the suspension, when the concentration or volume fraction of the free polymer ( p) exceeds a critical value denoted by p. Asakura and Oosawa reported the first quantitative analysis of the phenomenon [96]. They showed that when two particles approach to a separation that is smaller than the diameter of the free coil, polymer molecules are excluded from the interstices between the particles, leading to the formation a polymer-free zone (depletion zone). Figure 14.16 shows this for the situation below and above. ... 

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