(440j) Rheological Characterization of Dilute Polydisperse Bubble Suspensions

This work aims to characterize the rheology of bubble suspensions and determine the effect of polydispersity. To generate the suspensions, we chose glycerol, viscous mineral oil and a solution of glycerol and Sodium Dodecyl Sulphate as ambient fluids, owing to their Newtonian behaviour and wide use in the pharmaceutical industry. The viscosities of the ambient fluids were measured and found equal to 1.38 Pa∙s, 9.44 Pa∙s, and 1.38 Pa∙s, respectively. Among the three background fluids, the mineral oil provided the most stable bubble suspensions. To form the bubbles, we used a 3 μm pore size metal ring sparger and a 1-μm porous PTFE membrane setup as aeration devices. The initial bubble sizes were further reduced by mixing the suspensions in a high shear mixer. Among the two aeration devices, the porous membrane setup achieved the highest air volume fraction, equal to 12.81%. We characterized the rheology of the liquid-bubble suspensions via simple shear tests. The bubble size and polydispersity were determined via microscopy followed by image analysis. For all three systems, we measured key parameters, such as surface tension, viscosity of the continuous phase, bubble size distribution, bubble Sauter Mean Diameter and gas volume fraction. Based on these quantities, we calculated the relative viscosity (η_r) and the capillary number (Ca) of the suspension. We defined Ca as the ratio between the deformation time of the bubbles and the time they need to relax and return to their initial spherical shape. We found that our suspensions were highly polydisperse with the bubble size distribution following the gamma distributions for parameters a=7.87 and b=1.30 respectively. Comparison of our experimental results with the literature suggested that polydispersity has a profound effect on the viscosity of the bubble suspensions. For monodispersed bubble suspensions it has been proved that for very low and very high Ca the relative viscosity remains constant, while at Ca around 1 a steep shear thinning behaviour is observed. Our results showed that the shear-thinning behaviour occurs over a range of capillary numbers spanning from below to above 1. We attribute this behaviour to the different bubble sizes, as bubbles do not achieve deformation simultaneously. Therefore, when larger bubbles have reached Ca=1 and their shear-thinning contribution begins to appear, smaller bubbles are still spherical and resist the flow.