Liquid condensed domain coalescence-induced modulated structure formation in Langmuir monolayers: monolayer meets topology via elasticity and geometry

Abstract

We have investigated the formation and evolution of various intriguing defect structures by coalescence of liquid condensed (LC) domains surrounded by the liquid expanded (LE) phase at the late stage of the first-order phase transition in myristic acid Langmuir monolayers. At high temperature, domain coalescence proceeds at a high area fraction of LC domains mainly through the formation and annihilation of either a precursor passage (bridge) in the contact region or a highly curved single defect line. At low temperature, coalescence events have occurred already at a low area fraction of LC domains and we have found that the coalescence of two boojum domains (in which the magnitude of the molecular tilt is uniform and the tilt direction varies continuously) produces the emergence of remarkable splay stripe structures with arrays of domain walls (in which the magnitude and direction of the molecular tilt vary as a function of position). The conditions for the emergence of stripes to occur have been examined through a combination of elastic continuum theory and simulated annealing Monte Carlo procedure. The competition between intrinsic molecular tilt variations originating from the head–tail asymmetry of the molecules and the preference of the molecular tilt azimuth to be normal to the LE–LC interface is enhanced by the rapid change in the boundary conditions along the contact line formed by coalescence of boojum domains, developing a spontaneous striped structure by the more optimal tilt–splay coupling.