Spreading of surfactant solutions over hydrophobic substrates

The spreading of surfactant solutions over hydrophobic surfaces is consider ed from both theoretical and experimental points of view, Water droplets do not wet a virgin solid hydrophobic substrate, It is shown that the transfe r of surfactant molecules from the water droplet onto the hydrophobic surfa ce changes the wetting characteristics in front of the drop on the three-ph ase contact line. The surfactant molecules increase the solid-vapor interfa cial tension and hydrophilize the initially hydrophobic solid substrate jus t in front of the spreading drop, This process causes water drops to spread over time. The time of evolution of the spreading of a water droplet is pr edicted and com pared with experimental observations. The assumption that s urfactant transfer from the drop surface onto the solid hydrophobic substra te controls the rate of spreading is confirmed by our experimental observat ions. (C) 2000 Academic Press.