The blooming transition in Langmuir monolayers and its microscopic origin

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The behavior of Langmuir monolayers of methyl octadecanoate, methyl nonadecanoate and methyl eicosanoate was studied by polarized fluorescence microscopy and Brewster angle microscopy in the neighborhood of a transition between two liquid-condensed (LC) phases. As the monolayer is cooled through the transition, the molecular tilt azimuth in six-arm star defects of the LC phase changes by 90° (from play to bend texture), a process called “blooming”. In other defects, the transition is seen as a change from textured distorted hexagons to rectangular domains in which the tilt direction is uniform. The changes in texture and morphology are consistent with a transition from a hexatic phase to a crystalline phase with herringbone order.