Transient behavior of the velocity profile in channel flow of a Langmuir monolayer

We have observed the surface pressure-driven flow of langmuir monolayers th rough a narrow channel by means of Brewster angle microscopy (BAM). The mon olayers, composed of long chain carboxylic acids, were studied in a "hexati c" liquid crystalline mesophase (L-2 phase). The velocity profile across th e channel demonstrated transient behavior; i.e., it evolved from the expect ed parabolic shape to triangular as a function of flow rate, location along the channel, and time elapsed after start-up of flow. These transient effe cts appear to be related to the total strain in the system and suggest the importance of elasticity in the monolayer flow response. For situations in which the velocity profile was triangular, the distribution of domain width s across the channel indicated that the degree of domain stretching was sys tematically less than expected from the amount of strain the monolayer had experienced. This implied that elasticity of domain boundaries and/or slipp age between domains resists domain stretching and may contribute to the obs erved non-Newtonian behavior of the monolayer.