(665b) Inverse Emulsion Stability in Shear Flow

Inverse emulsions (IE) are of paramount importance in several industrial areas, such as cosmetics, pharmaceutics and oil industry. Many studies focused on their preparation, stabilization and destabilization mechanisms¹. Despite this, no investigation has been reported in the literature on the shear-induced aggregation of IEs. Therefore, the aim of this work is twofold: i) to explore the possibility of the shear-induced gelation in IEs and ii) to identify the key parameters that regulate the observed destabilization and gelation phenomena.

The employed IE systems consist of soft, water-swollen polymer-based particles at large particle contents, dispersed in an organic solvent and stabilized by a mixture of steric surfactants. It is found that at a fixed shear rate, the shear-viscosity of the IE systems first decreases with time (shear thinning), and then, after reaching a local minimum, it increases explosively. Cryo-SEM experiments reveal that along the shear history, the IE systems undergo competition between coalescence and aggregation, leading eventually to fractal gelation.

With respect to that of non-coalescing colloidal systems, the shear-induced gelation² of the IE systems results clearly from a different mechanism: coalescence slows down the gelation process by reducing the total occupied volume of the clusters. Only when the aggregation rate becomes faster than the coalescence rate, the gelation phenomenon becomes possible. Through the shear-induced gelation, we have identified the main parameters affecting the shear-stability of the IE systems, and their effects can be well interpreted by considering their role in altering the competition between coalescence and aggregation kinetics.

Literature:

1.     I. Capek, Adv. Colloid Interface Sci., 2010, 156, 35-61

2.     A. Zaccone, H. Wu, D. Gentili and M. Morbidelli, Physical Review E, 2009, 80