Fire Safety Science Digital Archive

The process of making foam for fire-fighting use is one in which an aqueous solution of a foaming agent or compound is mixed with a chosen proportion of air, and the whole is worked together until the typical "bubble structure" of foam appears. The properties of the resultant foam will depend upon the type and concentration of foaming agent, the degree of aeration or "expansion" and the amount of the applied energy converted into surface energy in the foam. This surface energy may be related (1) directly to the "stiffness" of the foam, as measured by its critical shear stress, and to its rate of drainage, properties which are both important to the fire-fighting qualities of the foam. It is of interest that in practical fire-fighting equipment, about 1 per cent of the applied energy is converted into surface energy, about 7 per cent into kinetic energy for projection of the foam, and the remainder is dissipated as heat. This note describes an investigation into the relationship between applied energy and critical shear stress, for foams produced from wetting agent and protein based compounds in a range of concentrations at two levels of expansion.