Effect of surfactants on shear-induced gelation and gel morphology of soft strawberry-like particles.

The role of surfactant type in the aggregation and gelation of strawberry-like particles induced by intense shear without any electrolyte addition is investigated. The particles are composed of a rubbery core, partially covered by a plastic shell, and well stabilized by fixed (sulfate) charges in the end group of the polymer chains originating from the initiator. In the absence of any surfactant, after the system passes through a microchannel at a Peclet number equal to 220 and a particle volume fraction equal to 0.15, not only shear-induced gelation but also partial coalescence among the particles occurs. The same shear-induced aggregation/gelation process has been carried out in the presence of an ionic (sulfonate) surfactant or a nonionic (Tween 20) steric surfactant. It is found that for both surfactants shear-induced gelation does occur at low surfactant surface density but the conversion of the primary particles to the clusters constituting the gel decreases as the surfactant surface density increases. When the surfactant surface density increases above certain critical values, shear-induced gelation and eventually even aggregation do not occur any longer. For the sulfonate surfactant, this was explained in the literature by the non-DLVO, short-range repulsive hydration forces generated by the adsorbed surfactant layer. In this work, it is shown that the steric repulsion generated by the adsorbed Tween 20 layer can also protect particles from aggregation under intense shear. Moreover, the nonionic steric surfactant can also protect the strawberry-like particles from coalescence. This implies a decrease in the fractal dimension of the clusters constituting the gel from 2.76 to 2.45, which cannot be achieved using the ionic sulfonate surfactant.