Dynamic effects in the stability of dispersed systems

Various motion-induced effects and their influence on properties of solid in liquid and gas in liquid dispersions are reviewed. First, it is discussed how presence of an interface (wall) can affect a particle motion under creeping flow conditions. The particle mobility near the interface depends mainly on the distance between the interface and the particle, the nature of the interface and the particle, the direction of particle motion relative to the interface orientation and fluid flow. The largest wall effects are observed in the case of solid particle moving towards solid interface where for example at the distance equal 110 of the particle radius its velocity is eleven times smaller than in unbounded fluid. A particle moving parallel to an interface is forced into rotational motion and the angular velocity of rotation increases as the gap width decreases. The presence of a particle deposited on the wall affects flow of other particles moving in its vicinity and it is a reason for the “blocking effects” observed in the deposition processes. It was shown that at higher Reynolds the wall effects can be neglected for the motion of bubbles during formation of gas in liquid dispersions, i.e. foams. On the other hand the motion induced disequilibration of surface coverages over the bubble surface has a pronounced effect on the probability of foam films and foam layer formation and stability. When the bubble reaches the interface the tendency to re-establish the equilibrium coverage by diffusion of surfactant increases the stability of the film while the drainage of the film leads to its rupture. The ratio of the rates of these two processes determines the magnitude of the stabilizing forces i.e. volume and stability of the foam layer formed. Changes of the solution foamability within homologous series of n-alkanols and fatty acids are reviewed and correlated with the changes of the effective elasticity forces, i.e. the elasticity forces which can be induced in foam films under an actual non-equilibrium surface coverages. It is demonstrated that neglecting a dynamic effects can lead to erroneous conclusions regarding importance of surface elasticity forces in stability of wet foams.