| The spreading behavior of a liquid on the surface of a solid substrate is greatly changed by the presence of a molecular layer of organic material (a surfactant) on the liquid surface. The surfactant reduces the surface tension of the liquid, depending on its concentration. When the liquid spreads, a gradient of surface tension is established, due to the nonuniform distribution of the surfactant concentration. The surface tension gradient can become the dominant force driving the spreading. The surface of the solid may also be modified by the adsorption of surfactant molecules. In this work, we study the spreading of water covered by a monolayer of valinomycin on a vertical glass slide, using an apparatus for Langmuir-Blodgett film deposition. For non-steady spreading, in which the slide is fixed and in contact with the monolayer covered water, a thin water film moves upward on the solid surface and finally stops. The velocity of the spreading and the final height of the film depend on the surfactant concentration on the water in the trough. For steady state spreading, in which the slide is continually lowered after it touches the water, the film approaches a constant length, which depends on the surfactant concentration and the velocity of the slide. In both steady and non-steady state situations, the spreading front is highly unstable: it bifurcates while spreading, forming tree-like patterns. We also studied the transfer of surfactant covered film between two Langmuir troughs over a bridge-shaped glass slide when the surfactant concentrations in the two troughs were maintained at different values. We applied the Landau-Levich theory to this problem, assuming the Deryagin-Landau disjoining pressure of water films on glass. The theoretical relationship between the rate of transfer and the surface pressure differential in the two troughs is in good agreement with the experiment. |
