LIQUID LENSES AT FLUID FLUID INTERFACES

In a previous study we have explored how the wettability of a spherica l solid particle resting in a fluid/fluid interface can be influenced by effects of line tension (tau) acting in the circular three-phase co ntact line around the particle. Here, we extend that study to consider possible effects of line tension on a small liquid lens resting in a liquid surface; the deformability of the lens predictably adds some ne w features. In the case of a lens we are interested in the way in whic h tau influences both the complete wetting of the subphase by the lens and the complete wetting of the lens by the subphase (i.e. engulfment of the lens). We propose a definition of a spreading coefficient (for the lens material on the subphase) which incorporates line tension. N egative 2 favours spreading and it is shown that it is possible in sys tems in which large lenses would not spread (e.g. dodecane on water at room temperature), the operation of negative line tension could cause spreading in lenses below a critical radius. For positive tau the beh aviour of a lens broadly mirrors that of a spherical solid particle. F or line tensions below a critical value tau(c) the lens can assume a t hermodynamically stable configuration in the interface. This happens w hen the Gibbs energy of the system with the lens at the interface is m ore negative than that for the system in which the lens exists (in the form of a spherical droplet) in the more wetting of the contiguous ph ases. For line tensions between tau(c) and tau(m), only metastable con figurations are possible; tau(m) is the line tension above which no st able configuration for the lens at the interface is possible. We consi der possible effects of line tension in surfactant systems of potentia l practical interest, and allude to the role of liquid oil droplets in the rupture of thin liquid films and hence in foam breaking.