Wetting Layers Close to Coexistence
A substrate immersed in a two component liquid mixture will preferentially adsorb one of the components. I describe in this thesis theoretical and experimental results on the smooth divergence of the thickness of this adsorbed layer as the system is brought close to two phase coexistence. I present a free energy functional which combines a phenomenological equation of state suitable for near critical binary liquids with a mean field treatment of bulk and surface ions. For the system carbon disulfide plus nitromethane on glass, the theory agrees well with experiment for suitable choices of theoretical parameters. Electrical conductivity measurements of the coexisting liquid phases confirm the choice of ion concentration values. By driving a stirred binary liquid mixture off coexistence with a linear temperature ramp, very small perturbations off coexistence are possible. I apply this method to the system perfluoromethylcyclohexane plus methylcyclohexane on Si wafers, measuring the adsorption with DC null ellipsometry. As the distance from coexistence decreases, the layer thickness smoothly increases and shows a crossover from adsorption dominated by the nonzero correlation length to adsorption dominated by dispersion forces, in agreement with theoretical predictions.
- Pub Date:
- INTERMOLECULAR FORCES;
- Physics: Fluid and Plasma; Physics: Molecular