Domain morphology in a two-dimensional anisotropic mesophase: Cusps and boojum textures in a Langmuir monolayer

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Abstract
Brewster angle microscopy (BAM) and polarized fluorescence microscopy (PFM) were used to observe domains of a tilted condensed phase in coexistence with an isotropic liquid phase in Langmuir monolayers of pentadecanoic acid. The orientation of the molecular tilt direction inside the domains can be described as a ‘‘virtual boojum’’ texture and a cusp appears in the domain boundary at the vertex of the boojum. The angle subtended by the cusp increases with the domain size for domains greater than 25 μm in diameter. All of these observations are at least qualitatively consistent with the recent predictions of a generalized Wulff construction for deformable materials [J. Rudnick and R. Bruinsma, Phys. Rev. Lett. (submitted)].