Fluorosurfactant retention in the foam blanket during gravitational drainage of an aqueous film-forming foam

A model aqueous film-forming foam (AFFF) comprising a fluorocarbon surfactant (6:2 FTSaB), hydrocarbon surfactant, solvent, and water was deployed on a heptane fire with the gravimetrically-drained foam solution collected at regular time intervals and analysed for surfactant concentration. Foam drainage behaviour within the model foam system was non-linear for both control (no fire exposure) and fire-exposed tests, as would be expected. However, the concentration of fluorosurfactant within the drainage solution was initially lower, increasing with time. This suggests that the fluorosurfactants are retained within the foam blanket for longer than other constituents of the model foam. This is proposed to result from the unequal probability of rupture in bubbles comprising the foam blanket: as the rate of foam collapse decreases with time, bubbles with lower stability burst until those with a higher persistence and higher surfactant content remain