Deposition of Oil Drops on a Glass Substrate in Relation to the Process of Washing.

The attachment of emulsion drops to glass substrates is investigated in relation to the redeposition of oil drops in the process of washing. It turns out that the drops of a surfactant-stabilized oil-in-water emulsion cannot be attached to an immersed glass plate simply by the buoyancy force. However, the same drops can be deposited on the plate when the latter is pulled out of the emulsion, i.e., when the drops are pressed against the substrate by a receding meniscus. We measured the amount of the oily deposit as a function of the pH, ionic strength, and composition of an amphoteric-anionic surfactant mixture. The enhanced oil deposition at low pH correlates with the domain in which the emulsion drops and the solid substrate bear opposite electric charges. This was established by zeta-potential measurements with oil drops and glass particles. The anionic surfactant brings negative surface charge to the oil droplets and suppresses the oil deposition on the negatively charged glass. With the increase of the fraction of the amphoteric surfactant in the mixture, the zeta-potential is converted from negative to positive, and the oil deposition grows almost linearly with the potential. In general, the deposition of oil drops by a receding meniscus is governed by an interplay of electrostatic and hydrodynamic factors. Copyright 2000 Academic Press.

Minimization of the Free Energy of Arbitrarily Curved Interfaces

The minimization of the free energy of a two-phase system with an interface of arbitrary curvature leads to an extremum (Laplace) condition containing the pressure difference, DeltaP, between the two sides of the interface. The expression for DeltaP is a function of the normal curvatures and of the resulting bending moments which are themselves functions of the normal curvatures, the mathematical form of which depends on the particular model for the interfacial bending energy that has been employed. On this basis, conclusions can be drawn about the equilibrium shape and curvatures of an interface, e.g., for bicontinuous microemulsions and vesicles. In addition, the pressure difference between the inside and the outside of surfactant-laden interfaces can be calculated. This pressure difference influences the work of formation of microemulsion droplets. A section devoted to the boundary conditions has also been included where in particular the case of a liquid meniscus attached to a cylindrically shaped solid surface is treated.