Breaking of Emulsions with Chemical Additives: Using Surrogate Fluids to Develop a Novel Theoretical Framework and Its Application to Water-in-Crude Oil Emulsions.

We investigate the role of adding a water-soluble surfactant (Tween 20) that acts as a demulsifier on the stability of water-in-dodecane emulsions stabilized with Span 80. Performing bottle test experiments, we monitor the emulsion separation process. Initially, water droplets sediment fast (∼10 min) until they become closely packed and form the so-called dense packed layer (DPL). The presence of the DPL, a long-lived metastable high-water-fraction (70-90%) emulsion separating bulk oil and water layers, slows down significantly the kinetics (∼105 min) of water separation. Once the DPL is formed, the ratio of the volume of separated water to the total water amount is called as water separation efficiency. We assume that the emulsion stability is reached when the coverage of the emulsifier surfactant exceeds 80% and use the ideal solution approximation. From that, we rationalize the water separation efficiency and the minimum demulsifier concentration required to maximize it, in terms of the mean droplet size, the surfactant concentrations, the total water volume fraction, and the adsorption strength of the water-soluble surfactant. Model predictions and experimental findings are in excellent agreement. We further test the validity and robustness of our theoretical model, by applying it successfully to data found in the literature on water-in-crude oil emulsion systems. Ultimately, our results prove that the efficiency of a demulsifier agent to break a W/O emulsion strongly correlates to its adsorption strength at the W/O interface, providing a novel contribution to the selection guidelines of chemical demulsifiers.

Effect of a Surfactant Mixture on Coalescence Occurring in Concentrated Emulsions: The Hole Nucleation Theory Revisited.

By conducting both a bottle test and isolate drop-drop experiments, we determine the coalescence rates of water droplets within water-in-oil emulsions stabilized by a large amount of Span 80 in the presence of Tween 20, a surfactant that acts as a demulsifier. Using a microscopic model based on a theory of hole nucleation, we establish an analytical formula that quantitatively predicts the coalescence frequency per unit area of droplets whose interfaces are fully covered by surfactant molecules. Despite its simplicity and the strong assumptions made for its derivation, this formula captures our experimental findings on Span 80-stabilized emulsions as well as other results, found in the literature, remarkably well on a wide range of water-in-crude oil systems.

Coalescence in concentrated emulsions: theoretical predictions and comparison with experimental bottle test behaviour.

Fusion between emulsion drops, also called coalescence, may be undesirable for storage or sought after depending on the desired application. In this latter case, a complete separation of the two liquids composing the emulsion is required. The same objective may be applicable to foams. We have performed bottle test experiments on a model system of water in oil (w/o) emulsion stabilized by high amounts of hydrophobic surfactant Span 80. We observe two regimes for emulsion separation: the first regime, which is fast and includes sedimentation of the water droplets, and the second regime, which exhibits a very dense and stable emulsion zone. We predict the initial thickness of the dense zone as a simple function of surfactant concentration and mean droplet size. From the assumption that the coalescence rate depends only on the area of the thin film between two contacted droplets, we quantitatively model the separation kinetics of the dense emulsion zone. Our results give rise to a simple method that allows measuring the coalescence frequency per unit area, only by monitoring bottle test experiments.

Geometric constraints alter cell arrangements within curved epithelial tissues.

Organ and tissue formation are complex three-dimensional processes involving cell division, growth, migration, and rearrangement, all of which occur within physically constrained regions. However, analyzing such processes in three dimensions in vivo is challenging. Here, we focus on the process of cellularization in the anterior pole of the early Drosophila embryo to explore how cells compete for space under geometric constraints. Using microfluidics combined with fluorescence microscopy, we extract quantitative information on the three-dimensional epithelial cell morphology. We observed a cellular membrane rearrangement in which cells exchange neighbors along the apical-basal axis. Such apical-to-basal neighbor exchanges were observed more frequently in the anterior pole than in the embryo trunk. Furthermore, cells within the anterior pole skewed toward the trunk along their long axis relative to the embryo surface, with maximum skew on the ventral side. We constructed a vertex model for cells in a curved environment. We could reproduce the observed cellular skew in both wild-type embryos and embryos with distorted morphology. Further, such modeling showed that cell rearrangements were more likely in ellipsoidal, compared with cylindrical, geometry. Overall, we demonstrate that geometric constraints can influence three-dimensional cell morphology and packing within epithelial tissues.