(51e) Isolating Non-Gravitational Ageing Phenomena in Emulsions Utilizing Microgravity and Diffusing Wave Spectroscopy

Emulsions are liquid-liquid dispersions with diverse use cases ranging from foods and cosmetics to pharmaceutical formulations. They are typically comprised of one or more liquids dispersed as droplets (inner phase) into a second immiscible liquid phase (outer or matrix phase) and surface stabilizing molecules, such as surfactants, or particles. Emulsions are generally categorized by their constituent phases in order from inner most phase to outermost phase. For example, an emulsion may be oil in water (O/W), water in oil (W/O) or, for more complex multiple emulsions, oil in water in oil (O/W/O). For a given composition, emulsions are further classified by their thermodynamic stability. Chemical or physical changes in an emulsion may cause destabilization. In general, there are four main mechanisms of emulsion destabilization: 1) gravitational separation (e.g., creaming or sedimentation), 2) coalescence, 3) flocculation, and 4) Ostwald ripening. Each mechanism has a unique time scale in which phase separation occurs, however, they typically occur concurrently. For example, flocculation may increase the effective emulsion drop size and increase the rate of creaming or sedimentation.

For many emulsions studied on earth, gravitational separation is typically the dominating mechanism of destabilization and can obscure the other three ageing processes. In collaboration with the European Space Agency (ESA), the PASTA mission studies emulsion stability in the condition of microgravity using the Soft Matter Dynamics (SMD) module – a diffusing wave spectroscopy (DWS) facility – in the Fluid Sciences Laboratory on the International Space Station (ISS). Emulsions are created using magnetic actuation of a piston in sealed sample cells containing oil, water, and surfactant. The dynamics of the emulsion are recorded via an overview camera, a line camera, and an avalanche photodiode. Photon correlation for DWS in both forward and backscattering is calculated. DWS provides volume-averaged insight into the displacements of emulsion droplets allowing extraction of droplet size, frequency of droplet rearrangements, and optical properties over the lifetime of an emulsion. Similar emulsions are prepared and studied on ground-based replicas of the SMD to provide complimentary data points and disentangle ageing mechanisms. We report trends in extracted physical parameters and a workflow to statistically analyze relevant effects of non-ionic surfactant concentrations. Additionally, we compare trends for emulsion formulations studied on the ISS and on earth to determine how the mechanisms of destabilization may be quantified using DWS.