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

Physisorbed surfactant 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...