Surface tension gradient driven spreading of trisiloxane surfactant solution on hydrophobic solid

The spreading of aqueous solutions of trisiloxane surfactant on hydrophobic surfaces has been studied extensively, but the underlying mechanisms are still being debated. Recently, we advanced the view that the initial high rate of spreading is driven by the surface tension gradient that develops spontaneously over the air–solution surface due to surfactant depletion caused by stretching of this surface. In this short paper, we substantiate this view with additional experiments. To understand the role of capillary forces, spreading experiments were conducted so that during the spreading of surfactant solution over the solid surface, the fluid displaced from the solid was an organic liquid instead of air as is the usual case. Through this scheme, the balance of forces at the three phase contact line was altered, while retaining the same surface tension gradient over the air–solution interface. Such experiments were also conducted on solid surfaces of different wettability, to examine the role of the solid–liquid interfacial tension on spreading rate. Our finding is that the initial rate of spreading is not influenced by the forces at the contact line or at the solid–solution interface, leading to the conclusion that the major driving force for spreading of trisiloxane surfactant solution on hydrophobic surfaces is the surface tension gradient over the air–solution surface. The occurrence of a maximum in spreading area with surfactant concentration is shown to be consistent with this mechanism. We also present a comparison between a simplified theoretical model and experiments to support the conclusion of surface tension gradient driven spreading.