Couette flow of two immiscible liquids between two concentric cylinders: The formation of toroidal drops and liquid sheaths

We describe the direct observation of deforming water drops in oil undergoi ng shear flow in a horizontal annular Couette cell. The drops assume a wide variety of highly reproducible structures depending on drop size, rotation speed, and flow history. These structures include toroidal rings of water around the rotating shaft and water sheaths, which, depending on experiment al conditions, can either expand to press against the inner walls of the ou ter stationary cylinder or contract to hug the outside of the rotating shaf t.