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

Surfactant transport through the three-phase contact line is studied in rel ation 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 hydrophili c plate into aqueous surfactant solutions. The adsorption at the liquid/sol id interface has been measured by ellipsometry, the wetting tension has bee n determined according to the Wilhelmy plate method, and the adsorption to the liquid/vapor interface has been evaluated from surface tension measurem ents. The relative importance of different adsorption modes could thereby b e grasped. For hydrophobic substrates, the dynamic wetting behavior is stro ngly affected by surfactant carryover through the advancing three-phase con tact 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 importa nt, the predominant transport mode being bulk diffusion.