Capillary rise of surfactant solutions

Abstract

The capillary flow of surfactant solutions in hydrophobic and hydrophilic capillaries has been studied. The results obtained are discussed in the light of surface tension relaxation and adsorption phenomena. Modes of surfactant transport and effects of surfactant adsorption, aggregation, and monomer–micelle interconversion on the capillary rise dynamics are further analyzed in the framework of a theoretical model. This provides a self-consistent and quantitatively correct picture of the experimental results. In the case of hydrophobic capillaries, two limiting types of dynamic behavior are adsorption-controlled rise and diffusion-controlled rise. In some cases, slow relaxation of adsorbed layers at the liquid/solid interface can cause a drift of the rise height over a long time under quasi-equilibrium conditions. A slow relaxation, as compared to the capillary rise rate, of the liquid/vapor interface may also produce a maximum in the rise s. time curve. This behavior was, for instance, seen for a surfactant solution penetrating into a hydrophilic capillary.