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. In order 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 fire retardants. The polyfluorinated components unique chemical structure renders this class of compounds particularly effective at reducing the transport of fuel vapors through the foam layer while also producing stable foams that rapidly spread on top of fuels. Unfortunately, the PFAS compounds in AFFF released in the environment can undergo chemical reactions that produce perfluorinated carboxylates or sulfonates that are bioaccumulative and harmful to wildlife and human health. In order to eliminate PFAS materials in AFFFs, effective alternative formulations for fire suppressants are needed. To accomplish this, a more thorough understanding of the role of the surfactant in foam formulation and fire suppression is required. For both systems, 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 Defense laboratories, will provide 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. The surfactant systems include those in Navy-procured cleaners found in bilge waters, provided by the Naval Surface Warfare Center Carderock Division (NSWCCD), as well as surfactant mixtures for potential alternative AFFF formulations, including siloxane-glucoside surfactant mixtures, provided by the Naval Research Laboratory (NRL). This work leverages droplet-based microscale approaches recently developed by the project team in the SERDP-supported WP18-1031 research effort, in order to characterize surfactant transport and interfacial adsorption timescales, and directly visualize film drainage and coalescence. The tasks will test the overarching hypothesis that the concentration and interfacial curvature-dependent timescales of soluble surfactant transport are deterministic in destabilization of emulsions and foams, wherein systems containing surfactants with higher interfacial adsorption, lower interfacial advection, and higher interfacial diffusion result in immobile (no slip) interfaces and slower film drainage times.

The work will result in mechanistic-level understanding of the role of Navy procured shipboard cleaning agents on bilgewater stabilization, toward improved shipboard treatment strategies. The work also seeks to establish the role of surfactant transport on foam stability in the presence of fuel vapors, toward improved alternative AFFF formulation and fire suppression performance. To that end, the project team will work closely with members from other SERDP emulsion and AFFF teams, for consistency in experimental protocols and model systems, and for data sharing and knowledge transfer. Each task area results in deliverables for improving shipboard treatment and new potential formulations, including new measurements, platforms, and reports. The work will yield new fundamental understanding of soluble surfactant transport at fluid-fluid interfaces.