In the literature, both the increase and the decrease of

ɛ_{G} were observed as the liquid viscosity,

μ_{L}, increased. All the apparently contradictory results can be explained by interpreting them in terms of the “

dual effect of viscosity over ɛ_{G}, as described by Besagni et al. [

11]—they found that

ɛ_{G} continuously (and non-linearly) increases when the MEG concentration increase, up to

c_{MEG} = 5% −

µ_{L} = 1.01 mPa·s, along with the contribution of small bubbles (

Figure 22a). Conversely, if the concentration is further increased from

c_{MEG} = 5% −

µ_{L} = 1.01 mPa·s to

c_{MEG} = 80% −

µ_{L} = 7.97 mPa·s,

ɛ_{G} decreases (

Figure 22b). For this last concentration, the

ɛ_{G} curve lies even below that obtained for pure water. The authors explained this behavior occurs because at low viscosities, the coalescence is limited, and the large drag force reduces the bubble rise velocity, causing an increase in

ɛ_{G}. When the viscosity increases, the tendency to coalescence prevails, creating large bubbles rising the column at a higher velocity, thus reducing

ɛ_{G}. In particular, the “

dual effect of the viscosity” has been clearly observed in other experimental investigations [

60,

61,

62,

150,

151,

152,

153]: Eissa and Schugerl [

150] (

d_{c} = 0.12 m,

H_{c} = 3.9 m,

d_{0} = 2 mm) showed that

ɛ_{G} first increases (

μ_{L} < 3 mPa·s), then decreases (3 <

μ_{L} < 11 mPa·s), and finally becomes roughly constant (

μ_{L} > 11 mPa·s). Bach and Pilhofer [

153] (

d_{c} = 0.10 m,

d_{0} = 0.5 mm) showed that

ɛ_{G} first increases (

μ_{L} < 1.5–2 mPa·s), then decreases (3 <

μ_{L} < 11–12 mPa·s), and finally becomes roughly constant (

μ_{L} > 12 mPa·s). Godbole et al. [

152] (

d_{c} = 0.305 m,

H_{c} = 2.44 m,

d_{0} = 1.66 mm) showed that

ɛ_{G} first increases up to

μ_{L} = 2.23–4.75 mPa·s, then it decreases (7.81 <

μ_{L} < 52.29 mPa·s, depending on

U_{G}), and finally becomes roughly constant (

μ_{L} > 52.29 mPa·s). Khare and Joshi [

151] (

d_{c} = 0.20 m,

H_{c} = 3.0 m,

d_{0} = 2.0 mm) showed that

ɛ_{G} first increases up to

μ_{L} = 4 mPa·s, and then it decreases for 4 <

μ_{L} < 10 mPa·s. Ruzicka et al. [

60] (

d_{c} = 0.14 m,

H_{0} = 0.2–0.8 m,

d_{0} = 0.5 mm) showed that

ɛ_{G} first increases for

μ < 3 mPa·s, and it decreases for 3 <

μ < 22 mPa·s. Olivieri et al. [

61] (

d_{c} = 0.12 m,

H_{c} = 2 m,

H_{0} = 0.8 m, needle gas sparger,

d_{0} = 0.4 mm) showed that

ɛ_{G} first increases up to

μ_{L} = 4.25 mPa·s, and then it decreases at higher viscosities. Concerning the remaining literature surveyed, the main results are described here. Please note that these apparently contradictory results can be explained by the above-mentioned criterion. For example, Yoshida and Akita [

47,

112] (

d_{c} = 0.152 m) reported that

ɛ_{G} varies with

μ_{L} in an irregular manner. Wilkinson et al. [

56] (

d_{c} = 0.15–0.23 m,

H_{0} = 1.2 m;

d_{0} = 7 mm,

d_{c} = 0.158 m,

H_{0} = 1.5 m,

d_{0} = 2 mm) found that a high viscous liquid phase (

μ_{L} = 21 mPa·s) causes a decrease in

ɛ_{G}. Kuncová and Zahradnik [

59] (

d_{c} = 0.15 m,

H_{c} = 1 m,

H_{0} = 0.5 m,

d_{0} = 0.5 mm) investigated the effect of

μ_{L} on the dynamics of bubble bed formation using several aqueous solutions of saccharose (1 <

μ_{L} < 110 mPa·s)—they found a decrease in

ɛ_{G} when the viscosity increased. Hwang and Cheng [

154] (

d_{c} = 0.19 m,

H_{c} = 2.5 m,

d_{0} = 1 mm) investigated the

ɛ_{G} structure in highly viscous Newtonian and non-Newtonian media and observed that

ɛ_{G} decreases when the viscosity increases. Zahradnik et al. [

8] (

d_{c} = 0.15 m,

H_{0} = 0.53 m,

d_{0} = 0.5 mm) found that moderate/high viscosities (3 <

μ_{L} < 110 mPa·s) decrease

ɛ_{G}. Yang et al. [

57] (

d_{c} = 0.15 m,

H_{c} = 1.7 m,

d_{0} = 0.7 mm) investigated the influence of the viscosity (1 <

μ_{L} < 31.5 mPa·s) on

ɛ_{G} by using a viscosity-increasing agent; they observed that

ɛ_{G} decreases when the viscosity increases. It is worth noting that the influence of the viscosity may be also described in terms of non-Newtonian behavior (and vice-versa). This point has been discussed by Besagni et al. [

11] by coupling the results obtained by Olivieri et al. [

61] with the “

dual effect of viscosity”; the non-Newtonian related stabilization of the homogeneous flow regime proposed by Olivieri et al. [

61], along with the “

dual effect of viscosity” relationship between the flow regime transition and the gas hold-up, help to explain the higher

ɛ_{G} for the non-Newtonian solutions observed by Godbole et al. [

152].