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The orientation dynamics of polymers in constrained geometries is considered through studies of monolayer films at the air−water interface. Here, in situ optical techniques are employed to probe flow orientation in monolayers of phthalocyaninatopolysiloxane dispersed in either docosanoic acid, 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine, or arachidyl alcohol. Compression of the polymer monolayer creates alignment perpendicular to the compression direction. A well-defined extensional flow is imposed in the monolayer to study the dynamics of flow-induced anisotropy. The orientation process obeys a strain-scaling law, indicating the absence of relaxation on the time scale of the flow.

The transient behavior of a polymeric nematic liquid crystal interface in extensional flows is studied both experimentally and theoretically. Monolayers of the hairy rod polymer phthalocyaninatopolysiloxane subjected to two-dimensional, transient, extensional flows are modeled with the two-dimensional analogue of the rigid rod model. The scalar order parameter and the director orientation are compared with the experimental observables. Two parameters appear in the model:  an average rotational diffusivity and the intensity of the nematic field. The average rotational diffusivity is determined by fitting relaxation experiments. The intensity of the nematic field, which is modeled with the Onsager potential, is determined by starting from the molecular parameters. A good quantitative agreement is obtained between experiments and theoretical predictions.

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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