The effect of surfactants on wicking flow in fiber networks

Abstract

Wicking flow in random fiber networks is examined for various pure liquids and surfactant solutions. Although the pore geometry of the fiber structures is extremely complex, it is found that all liquids used obey the rate law of the simple Lucas—Washburn theory. Imbibition of surfactant solutions is found to differ significantly from that of pure liquids of equivalent surface tension, indicating depletion of surfactant from the wetting front by adsorption onto the fiber surfaces during penetration. For fiber networks which are wet out by the imbibing liquid, wicking rate is directly proportional to an effective surface tension at the meniscus. The difference between this wicking-equivalent tension and the equilibrium surface tension varies between different surfactants. Fiber structures which are partially wet exhibit penetration rates proportional instead to the adhesion tension, and for surfactant solutions, there are differences between the wicking-equivalent and equilibrium values. Differences in wicking performance among the surfactants are attributed to differences in both the extent of their adsorption at the fiber surfaces and their diffusivities.