On the spreading of liquids in the presence of surfactants

The identification and means for determining a predictive parametrization for the dynamics of liquids spreading on solids in the presence of surfactants are presented. The presence of surfactants poses a significant departure with respect to modeling the spreading of pure fluids. It is shown that, when surfactants are present, the traditional hydrodynamic assumptions predict an infinite force exerted by the fluids on the solid surface. However, the methods that successfully removed this singularity in the case of pure fluids, are not adequate here. This is due to the surfactant's ability to transfer between the fluid-fluid interface and the solid surface during spreading.;The material parameters that make the hydrodynamical problem well posed are identified; they are the dynamic contact angle and transfer ratio of surfactant. They are assumed to depend on the velocity of spreading and the local surfactant concentration in the vicinity of the moving contact line. These relationships are not amenable to a direct measurement. Therefore, in order for the parametrization to have predictive power, we devised an indirect way for the determination of the constitutive relationships. The fluid mechanical analysis of a feasible experiment provides the necessary explicit relations between macroscopically measurable quantities and the desired constitutive relationships.;The model was applied to the deposition of a Langmuir-Blodgett film. We show that--due to viscous effects--the surfactant concentration on the liquid interface can not be constant throughout the interface. The evaluation of viscous effects on the surface tension of the surfactant-covered liquid interface allows the prediction of collapse and rarification of the monolayer spread on the liquid interface--important in the performance of the deposition. The sensitivity of the deposition to the two modeling parameters is put into evidence.