Fluid Dynamics of Langmuir Films (original) (raw)

Aps March Meeting Abstracts, 1996

Abstract

The fluid dynamics of fatty acid monolayers subjected to extensional flows are described. The polydomain structure of docosanoic acid is examined using Brewster angle microscopy and it is demonstrated that both molecular orientation and domain distortion can be measured in situ. The extensional flows are generated using a four roll mill that is placed through monolayers residing at the air-water interface situated on a Langmuir trough. Data are presented on two separate phases: the liquid expanded and the liquid condensed phases. Of these two phases, only the structure of the liquid expanded phase is observed to deform affinely with the imposed flow. The liquid condensed phase, on the other hand, produces a shear-banded structure with the bands oriented at plus and minus 45 degrees relative to the axis of strain in the flow. The shear bands contain molecules with a tilt axis that is orthogonal to the imposed strain. Ultimately, the shear bands grow and the entire surface eventually contains molecules that are oriented orthogonal to the stretching direction. This process is reversible, and changing the direction of the flow causes a rearrangment of the molecular tilt direction.

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