The dynamics of surfactant-stabilized multiphase systems, including oil-in-water (O/W) emulsions and aqueous film-forming foams (AFFF), have direct impact on the overall characteristics of shipboard bilge wastewaters and replacement firefighting foams. Bilgewater is oily wastewater emulsion found in the lower chamber of ships, contaminated with oils, fuels, grease, and detergents used on-board. However, any water containing more than 15 parts per million of oil cannot be discharged overboard. Instead, the emulsion must be stored or treated shipboard. To improve the water treatment processes and increase the volume of water that can be discharged, improved fundamental understanding of the role of fuels, oils, and detergent surfactants in shipboard emulsion destabilization is required. Likewise, per and polyfluoroalkyl substances (PFAS) compounds are widely used as surfactants in AFFF for firefighting. The PFAS components’ unique chemical structure renders this class of compounds particularly effective at suppressing fires. However, there is a transition to fluorine-free foams taking place due to the negative aspects of PFAS. To eliminate PFAS materials in AFFF, effective alternative formulations for fire suppressants are needed, enabled through a more thorough understanding of the role of the surfactant at fluid interfaces and this effect on foam formation and fire suppression.

For both emulsions and foams, determining the dynamic role of soluble surfactant mixtures in stabilizing fluid-fluid interfaces is essential for improving bilgewater treatment and firefighting foam formulation. This effort, in close collaboration with Department of War (DoW) laboratories, provides new mechanistic-level understanding of factors that govern emulsion and foam destabilization, through new measurements and models of surfactant transport to, at, and along the fluid-fluid interfaces. The three task areas use a unique combination of microscale measurement platforms with adsorption isotherm and thin film models, to study multiphase destabilization processes at length scales most relevant to the chemically stabilized O/W emulsions and film forming foams found and used shipboard. This work leverages droplet-based microscale approaches recently developed by the project team in the SERDP-supported WP18-1031 research effort, to characterize surfactant transport and interfacial adsorption timescales, and directly visualize film drainage and coalescence.

The work has resulted in new measurements and new models of soluble surfactant transport to liquid-liquid interfaces, particularly in the presence of additives and mixtures. The key scientific findings that crossed Task Area efforts provide new insights into compositional effects in the stability of interfaces and thin liquid films present in foams and emulsions. It was found that the faster surfactant in synergistic surfactant mixtures dominate the dynamics of adsorption across interface type, curvature, and temperature. It was also shown that the polydispersity of commercial surfactant molecular weights can cause even single surfactant systems to behave as a mixture, with implications in their mechanism for stabilization. Additives also played a key role in either stabilizing or destabilizing interfaces, largely through the modification in the interfacial tensions and the surface coverage. The work explored the role of viscosity ratio and surfactant concentration on thin film stability and has provided a mechanistic explanation for the phenomenological ‘Bancroft’ rule in emulsion science. These results and others have been published in ten peer-reviewed publications, with more currently in review.

The work resulted in new mechanistic-level understandings of the role of surfactants from shipboard cleaning agents on bilgewater stabilization, to inform decisions in bilgewater management. The U.S. Navy is currently developing recommendations for bilge cleaners that prevent the generation of stable bilgewater emulsions or negatively impact their treatment, and insights gained on surfactant transport in bilgewater settings is being utilized to support a Naval Sea Systems Command funded investigation of commercial bilge cleaners (NSWCCD-63-TR-2020/38) and the test conditions for oil pollution abatement equipment. The work also results in new insights into surfactant additives for firefighting foams, with implications in DoW firefighting foam transition efforts. Despite the recent (January 2023) change in specifications, few products currently meet the demanding needs of the military facilities to maintain safety and survivability. The research here has pushed the understanding in surfactant synergism forward and provided new attention to bulk and interfacial properties to access synergism in surfactant mixtures and providing the foundational knowledge for follow-on SERDP project, WP24-4147, on design principles for surfactant synergism. (Project Completion - 2025)