Assessing the arrest of coalescence due to Marangoni effects in flowing emulsions using population balance.

The conditions for coalescence arrest due to Marangoni effect of surfactant enriched emulsions flowing through a microfluidic device are analyzed. For that aim, we develop a Population Balance Equation model that allows the quantification of coalescence occurrence for emulsion systems at different surfactant concentration making use of isotherms. Besides, our model requires three parameters with physical significance to describe the behavior of the emulsions under shear to include Marangoni flow. The results were benchmarked by comparison to microfluidic experiments reported in the literature. Hydrodynamic coupling was observed at intermediate shear rates wherein coalescence was favored compared to the kinetics without surfactant. The coalescence kinetics was found to depend on the intertwined roles of both surfactant coverage and flow properties. These results represent a step towards the use of Population Balance Equation models not only for the study and prediction of conditions leading to coalescence arrest determined by either the external shear, repulsive barriers or Marangoni based forces but also for product design and control.

Intertwining Roles of the Disperse Phase Properties during Emulsification.

The combined effect of viscosity ratio, interfacial tension, and disperse phase density on the process of droplet formation during emulsification was evaluated. For that aim, emulsification by ultrasonication of oil/water systems with viscosity ratios between 1 and 600, with and without surfactant was performed. The time evolution of the average droplet size was estimated by dynamic light scattering measurements. For viscosity ratios between 1 and 200 in the presence of surfactant, our results partly reproduce those of the intriguing U-type reported in the literature. Beyond that range, the droplet size decreases, as the viscosity ratio rises. For surfactant-free systems, the size is slightly affected by the increase in viscosity. This complex scenario is analyzed in terms of both the individual and intertwined roles of interfacial tension, viscosity, and density ratios: (1) if the interfacial tension dominates, the droplet rupturing process is independent of its internal properties, and inversely, (2) if the interfacial tension is low, the internal properties play a major role in the rupturing of the droplet. Finally, we identified a scenario in which the retarded addition of surfactant leads to emulsions with a stability similar to those with the surfactant added at the beginning, saving energy and time.

Surface Behavior of BSA/Water/Carbohydrate Systems from Molecular Polarizability Measurements.

The effect of the presence of glucose and sucrose on the nonintrinsic contribution to partial molar volume ⟨Θ⟩ni of bovine serum albumin (BSA) is determined by means of static and dynamic electronic polarizability measurements. For that aim, a combined strategy based on high-resolution refractometry, high exactitude densitometry, and synchronous fluorescence spectroscopy is applied. Both static and dynamic mean electronic molecular polarizability values are found to be sensitive to the presence of glucose. In the case of sucrose, the polarizability of BSA is not appreciably affected. In fact, our results revealed that the electronic changes observed occurred without a modification of the native conformation of BSA. On the contrary, a nonmonotonous behavior with the concentration is observed in presence of glucose. These results advocate the influence of the electronic polarization on the repulsive and attractive protein-carbohydrate interactions. An analysis using the scaled particle theory indicates that the accumulation of glucose on the protein surface promotes dehydration. Inversely, hydration and preferential exclusion occur in the vicinity of the protein surface for sucrose-enriched systems.