Fire Safety Science Digital Archive

This paper investigates the dynamic surface tension of aqueous film forming (AFFF) and fluorine-free (FF) foam solutions by pendant drop tensiometry. Additional experiments are conducted to measure the interfacial tension between the foam solutions and two hydrocarbon liquids, n-heptane and diesel oil. The pendant drop technique yields both surface and interfacial tension at time scales ranging from seconds to hours, being limited at the lower end by the time necessary to form a droplet, and at the upper end by the evaporation of the droplet’s material. The results indicate that the surface tension of AFFF formulations, diluted at the design concentration, rapidly reaches its equilibrium value. However, the approach to equilibrium is slowed down by additives, such as xanthan gum, present in the alcohol tolerant concentrates (ATC-AFFF). FF solutions show a slower approach to static values than AFFF formulations. At the design dilution, and at room temperature, these static values are in the order of 27 mN m-1, as compared to 16 mN m-1 for AFFF solutions. The measurements of the interfacial tension indicate rapid attainment of equilibrium for both AFFF and FF formulations, with the static values of around 2 and 0.9-2.5 mN m-1, for AFFF and FF, respectively. With the surface tension of n-heptane and diesel oil of 20.1 and 28.3 mN m-1, the spreading coefficient of FF formulations for the present systems varies between –9.4 and 0.6 mN m-1. These results indicate that film formation does not play a role during fire suppression by FF foams.