Figure 7 shows the void fraction from the tested conditions in the present work, there is a direct correlation between void fraction and gas superficial velocity. It was argued in the previous section that regime shift from homogeneous to heterogeneous operation regime can be identified from higher order statistics and probability density plots. Here, it was attempted to investigate the regime change at similar gas superficial velocity (USG = 27.6 mm/s) using void fraction data. Figure 7 shows that above USG = 27.6 mm/s, void fraction (ε ) deviates from the linear trend with gas superficial velocity, which indicates that the homogeneous regime was no longer present. Figure 7 also shows that for the highest viscosity tested (G1) the void fraction levels off after U_SG = 27.6 mm/s. Detailed observations of the bubble column showed that the void fraction in G1 tests levels off due to a significant drop in gas residence time (data not shown). In these cases, the relatively high gas superficial velocity results in formation of slugs from bubble coalescence near the sparger; these slugs are unstable and travel significantly faster than bubbles and, consequently, the gas residence time decreases.²² However, in the rest of the cases tested in Figure 7 (i.e. G3 and water) the void fraction increases with gas superficial velocity.