Unstable topography of biphasic surfactant monolayers

We study the conformation of a heterogeneous surfactant monolayer at a fluid-fluid interface, near a boundary between two lateral regions of differing: elastic properties. Thf monolayer attains a conformation of shallow, steep "mesas" with a height difference of up to 10 nm. If the monolayer is progressively compressed (e.g., in a Langmuir trough), the profile develops overhangs and finally becomes unstable at a surface tension of about K(deltac(0))(2), where deltac(0) is the difference in spontaneous curvature and K a bending stiffness. We discuss the relevance of this instability to recently observed folding behavior in lung surfactant monolayers, and to the absence of domain structures in films separating oil and water in emulsions.