Per- and polyfluoroalkyl substances (PFAS)-based aqueous film-forming foams (AFFF) can hold 135% - 395% of fuel in their matrices before the resulting three-phase foams (TPF) become flammable; with this hold-up defined as (mass of emulsified fuel)/(mass of foam solution prior to aeriation) × 100%. During firefighting of hydrocarbon fires, when the foam streams collide with surfaces of hot fuels, the fuel pick-up of AFFF amounts to 50% - 90%, i.e., fuel pick-up < limit of flammability. For this reason, blankets of AFFF show remarkable resistance to ignition. The situation is much different for fluorine-free firefighting foams (F3s). The flammability limit of the TPF is only 20% - 25%. While the solutions of F3s are not as good emulsifiers as those of AFFF, it is estimated that these foams can pick up between 20% and 50% of fuel during their forceful applications, based on the rheological properties of F3s. Thus, for F3s, fuel pick-up ≈ limit of flammability. This means that blankets of F3 can ignite, with F3 failing to control tank fires. This realization is, unfortunately, supported by experience from both airport rescue and fire fighting and suppression of fuel-in-depth fires. From these perspectives, the objective of this submission is to develop improved formulations of F3s with the limit of flammability of their TPF increased to 60%. This target has been set to exceed the maximum pick-up of fuels by these foams by 10%.

This investigation unifies experimental and computational approaches to elevate the flammability limit of TPF made from formulations of fluorine-free AFFF. (Task 1) Using the apparatus developed in the pilot study, the project team will measure the fuel pick-up of (i) at least three commercial formulations of F3 (Commercial F3), (ii) the so-called RefAFFF (C6 PFAS-based formulation) developed by the Naval Research Laboratory (NRL), and (iii) new Reference F3, which will be formulated by the project team in this project, and (iv) emulsifier-augmented Commercial and Reference F3. The project team will also determine the flammability limit of the TPF, and will measure interfacial properties, foaming, and foam stability. (Task 2) It is hypothesized that emulsifier-like surfactant molecules embedded in the fuel/foam-solution interfaces will provide extra separation between foam solution and short-chain hydrocarbon molecules (C8-C10), typical of gasoline, making TPF less flammable and more stable. To understand this behavior at a nanometer scale, the project team will perform (i) classical molecular dynamics simulations and will deploy the so-called umbrella sampling to calculate the Helmholtz free energy for the diffusion of fuel molecules through the interfaces in conjunction with thermodynamic integration and molecular mechanics Poisson-Boltzmann surface area computations, and (ii) neutron and X-ray reflectivity studies at the Australian Nuclear Science and Technology Organization at Lucas Heights near Sydney. The latter includes processing of the measurements acquired at the end of 2023 and an application for new access to the Lucas Heights facility. (Task 3) The project team will incorporate emulsifiers germane to those used in enhanced oil recovery and in ammonium nitrate emulsion explosives into novel formulations of firefighting foams (emulsifier augmented F3). (Task 4) The project team will perform suppression experiments at Charles Darwin University (CDU) and NRL. They will investigate bulk rheology of concentrates and foam solutions at CDU and quantify the resistance of foams to fuel diffusion at NRL. This will ensure that they can be deployed by the present fire-suppression systems.

This project is expected to deliver:

i. New measurements and a new model of fuel pick-up by three Commercial F3s (two F3s on Qualifying Product List – SOLBERG MIL-SPEC and ECOPOL A3+ MILSPEC, and Angus JetFoam ICAOC/National Foam AvioF3 Green), Reference F3 and RefAFFF.

ii. New data on flammability of three-phase foams and new atomistic representation of fuel transport across the fuel/foam-solution interface.

iii. New technologies for incorporating emulsifiers in foam concentrates, as emulsifiers are incompatible with surfactants normally used in firefighting foams.

iv. Emulsifier augmented F3, to improve the foam performance due to more effective separation of fuels and foam solution at the atomic level, hence increased resistance to fuel diffusion.

v. Rheological characterization of foam concentrates and foam solutions, to provide data for designing delivery systems for the novel foam concentrates and foam solutions.

vi. Training of one Ph.D. student.

vii. Three peer reviewed articles in Q1 journals.