Stratified thin-film flow in a rheometer

When two immiscible layered fluids are present in a rheometer, interfacial distortions driven by the centripetal pressure gradient can modify torque measurements and induce dewetting. In particular, we examine the steady-state interface shape of a thin film coating a stationary substrate beneath a second immiscible fluid that is driven by a rotating parallel-plate or cone. An asymptotic analysis of the interfacial distortion for the parallel-plate flow is compared with numerical solutions for both the parallel-plate and cone and plate configurations. We develop asymptotic criteria for dewetting of the thin film as a function of fluid and flow properties, and show that significant interfacial distortion and dewetting can occur due to secondary flow effects even at low Reynolds numbers. The distortion of the interface can result in increased or decreased torque measurements depending on the viscosity and density ratios between the two fluid layers. We relate these effects to recent experimental studies on liquid-infused rough media and discuss the stabilizing effect of surface microstructure.