CHANNEL FLOW IN A LANGMUIR MONOLAYER - UNUSUAL VELOCITY PROFILES IN ALIQUID-CRYSTALLINE MESOPHASE

We have observed the surface-pressure driven flow of an arachidic (eic osanoic) acid Langmuir monolayer through a narrow channel using Brewst er angle microscopy. By following distinctive features of the monolaye r domain morphology we determined the velocity profile across the chan nel for various values of surface pressure over a wide range of flow r ates. At low surface pressure within the L-2 mesophase, the velocity p rofile is parabolic for low flow rates. This implies that the surface viscosity dominates the coupling to the aqueous subphase as a source o f 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 (greater than or equa l to 20 mN/m) the velocity profile is again parabolic for low-flow rat es. However, as the flow rate is increased the velocity profile is obs erved to gradually sharpen, eventually becoming triangular. The critic al shear rate for the onset of this flow profile is 0.2 s(-1). In a ty pical fluid, such a profile would indicate sheer 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 rheolo gy at pi = 20 mN/m suggests the presence of a monolayer phase transiti on.