Flow visualization experiments were carried out in a mixing research rig (Fig. 1) consisting of a 190 mm diameter cylindrical tank with a flat bottom placed inside a rectangular outer acrylic tank. The outer tank was filled with tap water to minimize the optical distortion. Sequential digital images were taken with a digital video camera, in order to reveal IMR structures and the flow pattern during the experiments. In order to capture the cross-sectional areas of the IMRs, a plane sheet of light passing through the 5-mm slits at the centre of the cardboards located on both sides of the tank illuminated the IMRs, as shown in Fig. 1. The light sources used were two 1000 W Arri IP23 lamps.