Comprehensive study into high-altitude effects on foam stability of AFFF: Drainage, coarsening, and film thinning behavior

Aiming to characterize the performance of aqueous film-forming foam (AFFF) fire suppressants under varying altitudes, with the goal of improving their practical utility in high-altitude settings. This study examined foam performance with a particular focus on stability, at five distinct altitudes ranging from 50 m to 4000 m. A systematic series of controlled experiments are carried out, and parameters such as drainage time, coarsening rate, film lifetimes, and liquid film thinning are quantitatively assessed. The results show that with increasing altitude, surface tension decreases. This is attributed to an increase in the number of surfactant molecules at the gas-liquid interface as the critical micelle concentration (CMC) rises. The decreased air density resulting from lower atmospheric pressure leads to a reduction in foaming ability. Moreover, liquid drainage increases significantly at higher altitudes, primarily due to greater foam water content and enhanced gas diffusion between bubbles and the surrounding atmosphere. Notably, the foam coarsening rate decreases with increasing altitude. The increase in foam water content, as well as the accumulation of surfactant molecules at the gas-liquid interface, reduces the efficiency of gas transfer between bubbles. The Marangoni effect is strengthened at higher altitudes, which improves the stability and prolongs the lifetime of the liquid film. In addition, the competing effects of liquid drainage and the Marangoni effect cause the height of liquid film to undergo cyclical fluctuations. This study provides practical guidance for the effective deployment of AFFF in high-altitude settings.