Collapse mechanisms of Langmuir monolayers.

When a two-dimensional (2D) film is compressed to its stability limit, it explores the third dimension via collapse. Understanding this 2D-to-3D transition is of great importance as it provides insight into the origin of defects in thin films. This review draws attention to a reversible folding collapse first discovered in model lung surfactant systems and explores the driving forces for this mechanism. The mode of collapse can be tuned by varying the mechanical properties of the film. I present a continuum elastic theory that captures the onset of the observed folding instability and use digital image analysis to analyze the folding dynamics. This article further explores factors that determine the maximum surface pressure a mixed monolayer can sustain and explains the observed phenomenon using the principle of rigidity percolation. The folding transition observed in lipid monolayers described here has also been observed in other systems, including monolayers of nanoparticles.