Molecular orientation in Langmuir monolayers under shear

The coupling between the structure of the tilted condensed phases of docosa noic acid monolayers (L-2, L-2', and Ov) and an external shear flow has bee n studied using Brewster angle microscopy. Generally, the coupling results in a reorientation of the alkyl tails, although different kinematics are ob served depending on the thermodynamic phase, shear rate, and surface pressu re. The coupling can result in continuous orientational changes or in abrup t reorientation of the molecular tilt angle. These molecular-level effects are connected to the macroscopic texture as well; shear can result in eithe r domain fragmentation or annealing. A detailed quantitative analysis of ph enomena reminiscent of the "flow alignment" and "tumbling" mechanisms found in nematohydrodynamics is presented. The analysis reveals that the kinemat ics of the reorientation is different from that in nematic liquid crystals. The underlying lattice plays a crucial role, and the observed phenomena ca n be explained with geometrical arguments.