Liquid lenses at fluid/fluid interfaces

Abstract

In a previous study we have explored how the wettability of a spherical solid particle resting in a fluid/fluid interface can be influenced by effects of line tension (τ) acting in the circular three-phase contact 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 new features. In the case of a lens we are interested in the way in which τ 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. Negative τ favours spreading and it is shown that it is possible in systems 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 τ the behaviour of a lens broadly mirrors that of a spherical solid particle. For line tensions below a critical value τ c the lens can assume a thermodynamically stable configuration in the interface. This happens when the Gibbs energy of the system with the lens at the interface is more 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 phases. For line tensions between τ c and τ m only metastable configurations are possible; τ m is the line tension above which no stable configuration for the lens at the interface is possible. We consider possible effects of line tension in surfactant systems of potential practical interest, and allude to the role of liquid oil droplets in the rupture of thin liquid films and hence in foam breaking.