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

Abstract

We have observed the surface-pressure driven flow of an arachidic ͑eicosanoic͒ 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 L 2 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 (у20 mN/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 0.2 s Ϫ1 . 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 ϭ20 mN/m suggests the presence of a monolayer phase transition. ͓S1063-651X͑97͒14409-9͔

Non-Newtonian Behavior of an Insoluble Monolayer: Effects of Inertia

“…As will be shown in this paper, measurements with another viscous monolayer that has a more complicated equation of state (and presumably phase behavior), namely hemicyanine, exhibits shear thinning and possibly other non-Newtonian behavior. Shear thinning behavior has also been seen in low Re experiments with other Langmuir monolayers (9).…”

“…Not only is the magnitude of the maximum velocity significantly less than the corresponding predictions (37 and 72% less than predicted for the 1.2 and 1.5 mg/m 2 cases, respectively), the shape of the velocity profile is qualitatively different. The flattening of the velocity profile is indicative of shear thinning of the monolayer film (see (11,9) for further examples of nonNewtonian shear thinning in other monolayers). The comparison with the computations shows that the phase transition at C ≈ 0.9 mg/m 2 , inferred from the equation-of-state, may demarcate Newtonian vs non-Newtonian behavior, at least at this Re.…”

Pressure-dependent surface viscosity and its surprising consequences in interfacial lubrication flows

“…The so-called "surface-slit" [33] or "canal surface" [34] viscometers were among the earliest methods to measure surface viscosity and involve a narrow channel through which a surfactant 023301-6 0.01 monolayer is pushed by applying a surface pressure gradient [6,31,[35][36][37]. These studies, however, assumed a constant surface viscosity and focused on quantifying the hydrodynamic coupling between the interface and the subphase.…”

The influence of lead (II) ions introduced into the subphase on the stability of mixed "polyamic acid + surfactant" monolayers and manufacturing of dye-containing Langmuir-Blodgett polymeric films

“…After 5 min soaking in aqueous buffer solutions with pH = 1.68 and 9.18, the C Pb /C N ratio decreases to 0.036 and 0.127, correspondingly. The probable reason is protonation of COO -groups in acidic region, leading, as usual, [17][18][19][20][21] to decay of their complex with metal ions, and hydroxo complexes formation at high pHs (see above).…”

“…[16][17][18] The phenomenon was re-examined and studied in details within the past decade. [17][18][19][20][21] The structure of a Langmuir -Blodgett film, transferred onto the substrate, depends on the quality of the monolayer on the interface and conditions of multilayer structures 657 The Influence of Lead (II) Ions Introduced into the Subphase Vol. 17, No.…”