Wetting effects due to surfactant carryover through the three-phase contact line

Surfactant transport through the three-phase contact line is studied in relation to the wetting-enhancing effect of surfactants. A specially designed experimental setup is used to simultaneously monitor interfacial adsorption and wetting tension during forced immersion of a hydrophobic or hydrophilic plate into aqueous surfactant solutions. The adsorption at the liquid/solid interface has been measured by ellipsometry, the wetting tension has been determined according to the Wilhelmy plate method, and the adsorption to the liquid/vapor interface has been evaluated from surface tension measurements. The relative importance of different adsorption modes could thereby be grasped. For hydrophobic substrates, the dynamic wetting behavior is strongly affected by surfactant carryover through the advancing three-phase contact line, which appears to be the dominant mode of surfactant transport to the solid/liquid interface. Conversely, for hydrophilic substrates, three-phase contact line carryover of surfactant does not appear to be so important, the predominant transport mode being bulk diffusion.