No-slip boundary condition switches to partial slip when fluid contains surfactant

Physisorbed surfactants can change the hydrodynamic boundary condition of oil flow from "stick" to "partial slip", provided that the shear stress on the wall exceeds a threshold level that decreases with increasing surface coverage of surfactant. To demonstrate this, Newtonian alkane fluids (octane, dodecane, tetradecane) were placed between molecularly smooth surfaces that were either wetting (muscovite mica) or rendered partially wetted by adsorption of surfactant (0.2 or 0.1 wt% hexadecylamine). The surface spacing was vibrated at spacings so large that the fluid responded as a continuum. The resulting hydrodynamic forces agreed with predictions from the no-slip boundary condition when flow rate, peak velocity normalized by surface spacing, was low but implied partial slip when it exceeded a critical level. In other words, the "slip length" depended on reduced velocity. When the reduced velocity was sufficiently high, a plateau shear stress was observed, ≈1.3 N m^-2 for 0.2 wt% hexadecylamine, but also showing some dependence on the fluid, being ≈20% higher when the fluid was octane rather than tetradecane. In other words, adsorbed surfactant molecules appeared to act as a somewhat rougher surface, the smaller the adjoining molecules of fluid. The magnitudes of the slip lengths were considerably less than in experiments with chemically modified surfaces of equivalent smoothness. This study points to a possible mechanism by which "friction modifiers" operate in oil and gasoline.