Unstable topography of biphasic surfactant monolayers

We study the conformation of a heterogeneous surfactant monolayer at a flui d-fluid interface, near a boundary between two lateral regions of differing : elastic properties. Thf monolayer attains a conformation of shallow, stee p "mesas" with a height difference of up to 10 nm. If the monolayer is prog ressively compressed (e.g., in a Langmuir trough), the profile develops ove rhangs 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 absenc e of domain structures in films separating oil and water in emulsions.