Dynamic surface effect on boiling of aqueous surfactant solutions

Boiling of aqueous surfactant solutions is known to be drastically different from that of pure water. The experimental results showed that a small amount of surfactant additive makes the nucleate boiling heat transfer coefficient, h_b, considerably higher, and that there is an optimum additive concentration for higher heat fluxes. Beyond this optimum point, further increase in additive concentration makes h_b lower. In this work, the effect of surfactant additive on boiling is attributed to the departure from equilibrium surface tension which is produced by extension of the vapor-liquid interface during growth and coalescence of vapor bubbles in the vicinity of boiling surface. The surface tension of a surfactant solution is higher than the static value because the surfactant component cannot diffuse to the adsorbed layer promptly. This dynamic surface effect can be expressed by the Y (≡c(dσ/dc)²) value which is shown to be a representation of the elasticity of an adsorbed film as well. The similarity between the increment of h_b due to the addition of surfactant and the Y curves indicates that the dynamic surface effect may play an important role in the boiling process of aqueous surfactant solutions.