Temperature and flow rate dependence of the velocity profile during channel flow of a Langmuir monolayer

Brewster angle microscopy was used to observe the surface pressure-driven flow of various fatty acid Langmuir monolayers through a narrow channel. In the tilted liquid crystalline phases (L-2, L-2', and Ov), the velocity profile was generally parabolic at low flow rates, evolved to a nearly triangular shape! at higher flow rates, and often returned to a parabolic shape at very high flow rates (when it was possible to observe them). In the L-2 phase, the critical flow rates for the transitions between flow profiles showed a marked decrease with increasing temperature. However, the critical flow rates were insensitive to temperature in the L-2' and Ov phases. The temperature dependence of these critical flow rates for each of the three chain lengths studied (octadecanoic, eicosanoic, and docosanoic acids) is nearly identical if one adjusts the temperatures by the typical 5 degrees C per methylene group in order to consider equivalent positions in the generalized fatty acid monolayer phase diagram. This suggests that the non-Newtonian flow behavior is specifically linked to the structures of the mesophases themselves and not, due to effects such as coupling to the subphase or a simple correlation to the polydomain structure of the monolayers.