We consider the stability of a long free film of liquid composed of two immiscible layers
of differing viscosities, where each layer experiences a van der Waals force between its
interfaces. We analyse the different ways the system can exhibit interfacial instability
when the liquid layers are sufficiently thin. For an excess of surfactant on one gas–liquid
interface the coupling between the layers is relatively weak and the instability manifests as
temporally separated rupture events in each layer. Conversely, in the absence of surfactant
the coupling between the layers is much stronger and the instability manifests as rupture
of both layers simultaneously. These features are consistent with recent experimental
observations
