Direct Visualization of Flow-Induced Anisotropy in a Fatty Acid Monolayer

Brewster angle microscopy is used to directly visualize the influence of an applied extensional flow on the domain structure and molecular orientation of a docosanoic acid monolayer at the air-water interface. At a surface pressure of 12 mN/m and a subphase temperature of 15 °C (L₂ phase), extensional flow causes domain elongation parallel to the extension axis. A frequency domain analysis of the Brewster angle images indicates that the domains undergo an affine deformation in response to flow. AT 20 mN/m (L₂phase), the flow modifies not only the domain structure of the monolayer but also the azimuthal orientation of the fatty acid molecules. This flow-alignment process is strain-rate dependent. Thus, flow can couple to the monolayer order over a variety of length scales.