Monitoring the Early Stages of Formation of Oil-Water Emulsions Using Flow Cytometry

Characterization of complex oil emulsions is critical yet challenging both in science and in many industrial applications. Here we demonstrate for the first time the use of flow cytometry as a fast method for characterizing complex, polydisperse oil-water emulsions. Owing to our interest in understanding how the presence of specific ions might affect the properties of oil-water emulsions including size, polydispersity, and complexity, we present a systematic study of oil emulsions in deionized water and various brines of different ionic strength. Forward scatter (FSC) and side scatter (SSC) intensities associated with detailed statistics were judiciously combined to provide a better understanding of these complex systems. We find that the type and concentration profiles of ions around the oil droplets affect significantly the properties of the emulsion. Weakly hydrated cations NH4+ and Ca2+ appear to be more effective in screening the charge of oil droplets compared to the monovalent Na+ and divalent Mg2+ ions, respectively. As a result, coalescence and formation of larger droplets are seen in the case of NH4Cl and CaCl2 compared to NaCl and MgCl2, respectively. In addition, weakly hydrated anions such as Cl- can come closer to the oil surface and, thus, decrease the effective screening that the Na+ ions provide as compared to SO42- ions, which leads to more stable emulsions in NaCl compared to Na2SO4. In addition to these specific findings, the work demonstrates the utility of the technique as a new tool for characterizing oil emulsions in a wide spectrum of fields ranging from food to oil and gas applications.