Surfactant-free emulsions of oil and water can be stabilized by colloidal particles with some wettability by both liquid phases. These so-called Pickering emulsions have long been appreciated for their good stability, processing properties, and lack of emulsifier toxicity; renewed interest in particles as emulsifiers has most recently been sparked by their use in stimulus-responsive emulsions and in emulsion-templated microcapsules. Despite these technological advances, our theoretical understanding of Pickering emulsions lags far behind: it is still impossible to predict reliably, from the properties of the liquids and the particles used, even the most basic features of the emulsified system, such as the type of emulsion (o/w or w/o) formed upon mixing. This project aims at establishing a simple, yet predictive model for the emulsifying properties of colloidal particles in a given liquid mixture.
Intellectual Merit: The main factors responsible for emulsion stabilization by solid particles are explored experimentally and theoretically. Specifically this project tests the hypothesis that accurate predictions of the "preferred emulsion type" in a three phase system and far-reaching conclusions about the achievable emulsion stability can be obtained by considering the interaction of individual particles with the oil/water interface ? provided that two commonly neglected electrostatic contributions to the free energy are accounted for: the interaction of charged particles near oil-water interfaces with their electrostatic image, and the energy stored in the dipole field around a charged particle trapped within the oil-water interface. The approach combines the experimental characterization of charging, wetting and far-field electrostatic interactions in oil-water-particle systems with theoretical modeling and experimental tests of model predictions for the achievable emulsion type and stability.
Broader Impact: Results of this project will expand our fundamental understanding of particle-stabilized emulsions with current applications ranging from food formulations and skin care creams to oil recovery, and anticipated uses e.g. in oil spill remediation or drug delivery via smart microcapsules. The science of emulsions encountered in everyday life is introduced to preschoolers through first graders in a predominantly (97%) African-American Atlanta school through hands-on "science workshops" in close collaboration with the classroom teachers. A "demo kit" including materials and instructions for the most successful demonstrations will be made available to the teachers for use in future classes. At the collegiate level, the PI's ethnically diverse and predominantly female research group promotes diversity by example. Research results from this project are incorporated directly in a graduate elective course on Colloids & Surfaces taught by the PI; research in progress is presented regularly in a Colloid and Soft Matter seminar organized by the PI, which brings together more than 25 research groups from different disciplines at Georgia Tech and Emory University.