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

Abstract

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 (L2‘ phase), 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.

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“…and Whang et al., where surface order has been generated by manually compressing trough barriers, while in our case the order occurs spontaneously because of closed bubble topology, bubble shape relaxation, and disproportionation process in the initial stages after foam formation 23. 24 Monte Carlo simulations of the phase behavior of two‐dimensional hard‐rod fluids also showed formation of 2D nematic phases,25 depending on the surface coverage rod aspect ratio. However all these results relate to flat 2D geometries, while the image sequence in Figure 2 clearly demonstrates the important interplay between curvature and rod‐length ratio and rod rigidity, which suggests that surface structures could be independently controlled and fine‐tuned by these parameters as well.…”

How Rigid Rods Self‐Assemble at Curved Surfaces

“…There is also a similarity with hierarchical structures of nanorods or nanowires [20][21][22] assembled by using the Langmuir-Blodgett technique, observed by Yang et al and Whang et al, where surface order has been generated by manually compressing trough barriers, while in our case the order occurs spontaneously because of closed bubble topology, bubble shape relaxation, and disproportionation process in the initial stages after foam formation. [23,24] Monte Carlo simulations of the phase behavior of twodimensional hard-rod fluids also showed formation of 2D nematic phases, [25] depending on the surface coverage rod aspect ratio. However all these results relate to flat 2D geometries, while the image sequence in Figure 2 clearly demonstrates the important interplay between curvature and rod-length ratio and rod rigidity, which suggests that surface structures could be independently controlled and fine-tuned by these parameters as well.…”

Alignment of Hexatic Langmuir Monolayers under Shear

“…The use of recent observation techniques, such as BAM, has allowed a closer analysis of the coupling between an imposed flow and the structure of Langmuir monolayers. In recent work, Fuller's group [5][6][7] showed that a strong coupling between external flow and the orientational order of the monolayer may exist, and that the nature of the coupling differs for different tilted condensed phases. By studying the L 2 , L 0 2 , and S phases, they found that, depending on the phase, the initially rich mosaic structure may be annealed by a shear flow, so that only two reflectivities are observed, which they relate to the tails being oriented either along or against the flow.…”