Predicting the long-term stability of depletion-flocculated emulsions by static multiple light scattering (SMLS)

Abstract

Predicting the gravitational stability of weakly flocculated colloidal dispersions is a key problem in product formulation. For these systems, destabilization can occur suddenly after several months of apparent stability. During this time there are no visible signs of imminent collapse. We show that the intensity of backscattered light changes slowly during the period of apparent stability, and that its rate of change at short times correlates with the time when the system collapses, the stability time. Collapse occurred as sedimentation for weak gels, but as syneresis for stronger gels, presumably because the gravitational stress was less than the yield stress in the latter cases. The light spot backscattered from a point source by a turbid colloidal dispersion has a universal shape. In general, its size depends on only one parameter: the mean photon transport length, l*. An optical scanner was used to measure simultaneously the stability time and l* for an emulsion stabilized by a range of concentrations of three polymers. It was found that the stability time correlates with the rate of change of l* after 1 day, up to visual stability times exceeding 1000 days. This correlation can be understood by the relationship between l* and the static structure factor. Several previous studies have shown that the static structure factor increases at small scattering angles during the stability time. The correlation can form the basis of a method to predict the stability time of weakly flocculated dispersions.

GRAPHICAL ABSTRACT

Keywords:

• Stability
• Sedimentation
• Light scattering
• Emulsion
• Flocculation

Acknowledgements

C.T. and G.B. acknowledge useful discussions with Luca Cipelletti.