Bubble coarsening dynamics in fluorinated and non-fluorinated firefighting foams

• Firefighting foams followed the self-similar bubble growth law known for SDS foam.
• Fluorinated foams coarsened slower than non-fluorinated foams.
• The evolutions of the bubble-size distributions depended on foam chemistry.
• Diffusion rate differences were not explained by surface tension differences alone.

We have quantified the dynamics of bubble coarsening in some commercial firefighting foams. These multi-component foams contain unique chemical formulations leading to different coarsening and drainage behaviors. We show that these firefighting foams, some of which contain fluorocarbon surfactants and one of which contains only fluorine-free ingredients, as well as single-component, relatively well understood, sodium dodecyl sulfate (SDS) foam follow the self-similar bubble growth law that predicts increasing average bubble size with time. Further, the experimentally measured effective diffusion coefficients for coarsening are smallest for fluorinated foams, followed by a non-fluorinated firefighting foam, followed by SDS foam, as expected based on differences in surface tension. However, the effective diffusion coefficients derived from experiments on multi-component foams are smaller than predicted by the classical theory, which considers several physical properties including surface tension and lamella thickness. In contrast, the measured effective diffusion coefficient for SDS foam agrees with the theory. Therefore, the commercial firefighting foams coarsened slower than theoretically predicted relative to SDS foam, even after accounting for differences in surface tension and initial liquid content

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