Determination of the shear and extensional rheology of bubbly liquids with a shear-thinning continuous phase

The effect of bubble volume fraction, I center dot, and bubble size on the rheology of rheology of kappa/iota-hybrid carrageenan gum solutions and bubbly liquids with I center dot a parts per thousand currency signaEuro parts per thousand 0.25 was studied in steady shear using a parallel plate configuration and in extension using a filament stretching device. Three different bubble size distributions were prepared using (1) planetary mixing (mean bubble size 90-130 mu m), (2) mixing followed by pumping through a symmetric syringe (25-38 mu m), and (3) as (2) but using an asymmetric syringe (12-20 mu m). Gum concentrations of 10 and 20 g/L, lying in the semi-dilute regime, were studied. The unaerated gum solutions are viscoelastic and could be modelled very well as a single-mode Giesekus fluid. The bubbly liquid behaviour in both shear and extension could also be modelled as a single-mode Giesekus fluid. The Giesekus parameters for the bubbly liquids were related to those of the unaerated solutions by monatonic increasing dependencies on I center dot. The low shear rate apparent viscosity followed the trend predicted by the Choi and Schowalter (Phys Fluids 18: 420, 1975) model for emulsions, while the mobility parameter and relaxation time were linearly dependent on I center dot. In these tests bubble size only affected filament extension noticeably, influencing the mobility parameter. Further work is required to explain the observations in terms of micromechanical models.