Channel flow in a Langmuir monolayer: Unusual velocity profiles in a liquid-crystalline mesophase

We have observed the surface-pressure driven flow of an arachidic (eicosanoic) acid Langmuir monolayer through a narrow channel using Brewster angle microscopy. By following distinctive features of the monolayer domain morphology we determined the velocity profile across the channel for various values of surface pressure over a wide range of flow rates. At low surface pressure within the L₂ mesophase, the velocity profile is parabolic for low flow rates. This implies that the surface viscosity dominates the coupling to the aqueous subphase as a source of dissipation and that the monolayer behaves as a Newtonian fluid. At extremely high shear rates, a flattened velocity profile is observed, similar to plug flow. At higher surface pressure (>~20mN/m) the velocity profile is again parabolic for low flow rates. However, as the flow rate is increased the velocity profile is observed to gradually sharpen, eventually becoming triangular. The critical shear rate for the onset of this flow profile is 0.2s^-1. In a typical fluid, such a profile would indicate shear thickening. However, measurement of the surface pressure drop along the channel versus flow rate indicates that macroscopic surface viscosity actually decreases with shear rate in this regime. The sharp change in interfacial rheology at π=20mN/m suggests the presence of a monolayer phase transition.