Detergent stabilized oil in water (OW) emulsions are ubiquitous. While in certain applications, long time stability of these emulsions is desired, it can also be a nuisance in other situations. Stable emulsions cause concerns in various Armed Forces bilgewater applications as the extraction of water from the oily wastewater is expensive and becomes technically challenging. Understanding the causes underlying the formation of such emulsions should provide strategies to suppress the formation of such emulsions altogether. Furthermore, when they are formed, water can be extracted from the emulsion inexpensively, and efficiently. While the main objective of this research was to develop the fundamental understanding of OW emulsification under turbulent and agitation free conditions, the project team also developed methods to thwart such emulsifications by taking advantage of the knowledge gained from the same studies.

Emulsification of an oil in salinized water was studied under turbulent and agitation-free conditions in the presence of a mixture of an ionic and a non-ionic surfactant. The properties of the air-water and the oil-water interfaces were investigated using the methods of du-Nouy ring, drop resonance vibrometry and Langmuir film balance that allowed pinpointing the relevance of certain interfacial properties in emulsification. Estimation of the droplet size and its distribution from the nanometer to micrometer range was carried out with optical microscopy, acoustic attenuation spectroscopy and continuous hydrodynamic flow fractionation. These measurements provided the platform for the comparison of the emulsion droplet size with those predicted from the fluctuation of the dynamic stress in the turbulent water via a capillary-hydrodynamic model. While such a comparison was reasonably meaningful for micron size emulsion droplets, production of nanometer size droplets was beyond such a rudimentary expectation. The project team thus carried out systematic investigations into other factors that contribute to emulsification under both agitated and agitation free conditions.

An important finding of these studies is that the infusion of air bubbles that profoundly enhance the hydrodynamic fluctuation produce mainly submicroscopic emulsion droplets, while a fluctuation inhibiting water-soluble polymer has the opposite effect. Furthermore, while a hydrophilic polymer dissolved in water enhances the ripening of the droplets with time, hydrophobic polymer in oil thwarts aging, plausibly by osmotic backpressure and interfacial stiffening, which, upon compression, acts against surface tension, thereby decreasing the chemical potential of the trapped oil molecules inside the droplet. These effects are similarly observed in spontaneous emulsifications, that is when a layer of oil containing the additives is deposited upon the surface of the aqueous phase in the absence of any external work input. Micro and/or Nano emulsions are formed when an organic liquid gently comes in contact with water in the presence of a surfactant, even with a positive interfacial tension. Many years of research made it clear that the driving force for spontaneous emulsification arises from the differences of the bulk chemical potentials of various components, which triggers various non-equilibrium coupled transport processes, such as diffusion and hydrodynamic fluctuation.

While extraordinary theoretical developments have taken place that attempt to describe the emulsification processes within the formalisms of global equilibrium and non-equilibrium thermodynamics, the local processes underlying the spontaneous emulsification, however, still remain elusive. In this research, the project team attempted to shed light on some of the local processes that involve the transfer of surfactant as well as molecular water from one phase to another (i.e. water to oil), subsequent formation of inverted emulsion and its evolution to oil-in-water emulsion as they cross the phase boundary. These processes lead to either strong or weak fluctuation of component concentration just below the interface that announces fast (athermal) diffusion of the emulsion droplets farther into the bulk of water. These studies have been able to identify the relevant interfacial and hydrodynamic parameters, careful controls of which should be valuable in thwarting emulsification in various practical settings.