Flow at the interface of two rotating fluids

Abstract

Recent studies discovered a discontinuity of the radial velocity (slip) at the interface of two immiscible rotating fluids. This work investigates how this phenomenon depends on densities and viscosities of fluids. A sealed vertical cylindrical container models a bioreactor. The rotating lid drives the flow while other container walls are stationary. As the rotation intensifies, the slip develops at the interface. A conjecture is that the slip occurs due to a difference in fluid densities and the centrifugal force. To test this conjecture, this experimental work uses fluids of close densities and reveals that the slip does not disappear even if the density difference is small, compared with that in prior studies. For comparison, this study also explores the case where the densities significantly differ while the fluid viscosities are close. The slip occurs in this flow as well. A new conjecture is that competing effects of the upper-fluid rotation and convergence near the interface also can cause the slip. This study also shows that the slip is a robust phenomenon occurring in swirling flows of various fluids.