Stabilizing mechanisms stratification

Film studies in the past decade have revealed the existence of another film stability mechanism If the continuous phase contains not only a small amount of surface active substances but also a "sufficient amount" of "small particles", these particles can form layers inside the draining film (see Fig. 6, right)[9,22-32]. As a result, such films thin step-wise, by several step-transitions (also called stratification) when at a step transition a layer of small particles leaves the film. 

The potentiality of hierarchical stratification of complex reactive systems, according to the characteristic times of the involved processes, makes it difficult to use direcdy thermodynamic tools as well as to apply the con cept of stability to very compHcated (in particular, biological) systems. The statistical approach to describe the behavior of a system that contains a large number of particles takes into account the instabihty of mechanical trajectories of individual particles. Indeed, any infinitesimally small distur bances in the particles motion can make it impossible to determine from the starting conditions the trajectory of even one particle s motion. As a result, a global instabihty of mechanical states of individual particles is observed, the system becomes statistical as a whole, and the trajectories of individual particles are no longer predictable. At the same time, the states that correspond to stable solutions of any dynamic (kinetic) problem can only be observed in real systems. In terms of a statistical approach, the dynamic solution of a particular initial state of an ensemble of particles is a fluctuation, while the evolution of instabihty upon destruction of this solution is a relaxation of this fluctuation. 

The formation of ordered microstructure in thin liquid films offers a new mechanism for the stabilization of foams. As a proof of microstructuring in real foams, Figure 17 shows a photograph of an aqueous foam system stabilized because of the stratification in the foam bubble lamellae. The practical importance of the film microstructuring is that the lifetimes of foams with stratifying films are much longer. 

Improved properties of commingled plastics via blend modification by reactive functionalization and compatibilization have been reported by several workers (58,65). This work is confined to a two-phase PE-PP morphology. In the two-phase immiscible PE-PP system, poor interfacial adhesion results in poor blend mechanical properties. The lack of stability in the morphology causes gross separation or stratification during later processing or use. Block and graft copolymers of the form A-B have been used as compatibilizers to improve interfacial adhesion and reduce interfacial tension between A-rich and B-rich phases to provide A-B alloys with improved and unique balances of properties. 

It was found that in each case the foam stability was effected by a completely different mechanism. In the first case, where the foam film containing oil is solubilized within the micelle to form a microemulsion, the normal micellar interactions are changed. It had been earlier demonstrated that micellar structuring causes stepwise thinning due to layer-by-layer expulsion of micelles and that this effect was found to inhibit drainage and increase the foam stability. Generally, this stratification phenomenon was found to be inhibited by the oil solubilized within the micelle which decreased the micelle volume (representing a decrease in the repulsion between the micelles). 

The resulting double-layer stratification is stable to classical Rayleigh-Taylor or buoyancy-driven instabilities due to differential diffusion mechanisms, such as double-diffusion or double-layer-convection scenarios, and thus it is ideal to isolate the pure effect of cross-diffusion on the system stability. 

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