Topological manifestations of surface-roughening collapse in Langmuir monolayers.

A transition from surface-roughening collapse to a random network collapse has been investigated in fatty acid Langmuir monolayers. In contrast to the random network crack pattern, the surface-roughening crack pattern grows on a much longer time scale and on a much shorter length scale. A change in the isotherm from a surface pressure "spike" to a "plateau" occurs at the transition. In the surface-roughening regime, the pattern is manifested by the emergence of a three-dimensional (3D) disordered stripe phase with locally aligned, anisotropic stripe clusters. The individual stripes coalesce less with each other even at a late stage of the collapse process and there is a characteristic length for the stripe width and the nearest neighbor spacing. The pattern includes a high density of topological defects such as single open ends, twin open ends, branches, and loops. The effects of shear deformation on the observed patterns are discussed.