Diffusing Wave Spectroscopy: Decoupling Physical Phenomena in Concentrated Systems

Emulsions are the formulated form of a wide range of personal care and consumer products. A mechanistic understanding of the stability of these materials depends on the ability to decouple destabilizing effects, such as evolution of mean droplet size, the mean settling/creaming velocity of the dispersed phase, and the microrheology of the matrix phase. Typically, these effects are estimated using spatial scanning and average intensity of transmitted light through the sample. Although this approach can be used to identify when unknown dynamics have slowed or stopped, mechanical movement is required, inhibiting fast sampling times. Diffusing wave spectroscopy (DWS), a multiple scattering adjunct to photon correlation spectroscopy, has long been used as a tool to study concentrated and turbid samples. Due to its non-invasive nature, it represents an important technique to extract the structure and dynamics of sensitive and transient systems. DWS measurements are made on concentrated suspensions and emulsions in forward and back-scattering modes. Measurements made over the lifetime of complex formulations allows for accurate determination of stability and aging mechanisms. DWS presents a method of understanding what dynamics are dominant and how they are affecting the stability of multiphase systems. Further, utilizing light decorrelation in DWS provides near instantaneous measurements to be made, gaining access to extremely small sampling times. We show the utilization of DWS to provide quantitative determination of particle radius and settling velocity of polystyrene and SiO2 suspensions. We then use a similar framework to explore the role of surfactant concentration on the evolution of creaming velocity and average droplet radius in short-time aging in emulsions of isopropyl myristate and aqueous surfactant solutions.