Formation of a thin circulation layer in a two-fluid rotating flow

Recent numerical simulations predict the formation of a thin circulation layer (TCL) near the interface in a steady axisymmetric swirling flow of two immiscible fluids. This phenomenon, of fundamental and practical interest, has not been detected in prior experiments. The current experimental study reveals that the TCL does develop, but in a way which has not been predicted. The flow is driven by the rotating lid in a vertical cylindrical container-a model of a vortex bioreactor. The centrifugal force pushes the upper fluid from the axis to the sidewall near the lid and the fluid goes back to the axis near the interface. This centrifugal circulation (CC) drives the anticentrifugal circulation (AC) of the lower fluid at a slow rotation. As the rotation speeds up, a CC cell emerges below the interface-axis intersection. It expands downward and occupies almost the entire lower-fluid domain while the AC shrinks into a thin ring adjacent to the interface-sidewall intersection. As the interface starts to visibly rise near the axis, a new AC cell emerges below the interface-axis intersection. This cell and the AC ring merge and form the TCL, attached to the entire interface from below. The TCL is observed in a wide range of the lid angular velocity, until the flow becomes unsteady and three-dimensional. The resulting multicellular flow pattern is favorable for efficient mixing in bioreactors.