Experimental investigation of interfacial hydrodynamics of swirling flow in the presence of insoluble monolayers

The coupling between a monolayer-influenced air/water interface and the bulk fluid was examined through a series of experimental investigations in a vortical flow. The interfacial coupling is a result of the bulk liquid's viscosity ensuring that the surfactant film at the interface has the same velocity as the bulk fluid at the interface. The coupling then formally comes about by balancing the shear stress in the liquid evaluated at the interface with that of the interface. The most widely used constitutive relation for determination of the interfacial stress is the Boussinesq-Scriven surface model, applicable to so-called Newtonian interfaces. The appropriateness of such a relation for vitamin K₁, which forms an insoluble monolayer on water was tested in an axisymmetric annular flow (with a rotating floor and a free surface) and was proven to be valid for the elastic and inviscid interfacial model in the range of monolayer studied. When the concentration of the monolayer was high enough, results showed that the interface was immobile in the radial direction while mobile in azimuthal direction and influenced solely by one interfacial property, normally interfacial shear viscosity (μ s). Application of such conditions introduced a new approach in measuring μ s, similar to the method introduced by Mannheimer & Schechter in 1970 (similar annular flow but operated in the Stokes flow limit) now affected by inertia. Benefits of the present approach include higher signal-to-noise ratio and lower time for measurement (due to higher Reynolds number). Furthermore, while comparison between measurements and available numerical predictions for monolayers of vitamin K₁ and stearic acid on water showed the applicability of a Newtonian interface coupled to the bulk flow, it was found that for a monolayer of hemicyanine the behavior changed drastically. Evidence of shear thinning (non-Newtonian behavior) and its association with a phase transition at a concentration of C ≈ 0.9 mg/m2 was also presented. In an attempt to study the problem in a more simplified geometry, the flow inside a stationary cylinder with a rotating floor and a free surface was studied. For the first time in an open cylinder, symmetry breaking of the floor boundary layer to a mode 4 rotating wave (beyond Re_crit = 2000) was observed. Extension of the present approach for interfacial hydrodynamics to the general case of soluble surfactants was also explored.