Contraction and expansion flows of Langmuir monolayers

Abstract

Langmuir monolayers consist of amphiphilic molecules at the air–water interface and can be modeled as two-dimensional fluids. We have performed 4 : 1 contraction and 4 : 1 expansion flows on arachidyl alcohol, a Newtonian monolayer, and on PODMA [poly(octadecyl methacrylate)], a non-Newtonian monolayer. Arachidyl alcohol shows no vortices in contraction flow, but exhibits very large inertial vortices in expansion flow. In contrast, during contraction flow PODMA exhibits large vortices that have little or no growth as a function of flow rate. The elasticity of PODMA tends to diminish the inertial vortices in expansion flow as a result of `die swell'. Normal stresses accumulated in the channel shear flow cause the fluid to expand into the downstream channel, pushing the vortices into the salient corner. These results are in qualitative agreement with planar contraction and expansion experiments performed on three-dimensional fluids. This correspondence in results between two-dimensional and three-dimensional fluids is encouraging because it allows one to predict the flow behavior of Langmuir monolayers based on previous results for three-dimensional polymers and demonstrates that extensional and nonlinear rheological behavior are important in these systems.