Comparison of the rheology of bubbly liquids prepared by whisking air into a viscous liquid (honey) and a shear-thinning liquid (guar gum solutions)

• We study the effect of a shear-thinning continuous phase on bubbly liquid rheology.
• We compare honey (near-Newtonian) and guar gum (shear thinning) bubbly liquids.
• Bubbles impart significant normal forces when honey-based liquids are sheared.
• Significantly larger normal forces are generated in guar gum solutions.
• Guar gum solutions containing surfactant give higher air content and larger bubbles.

Many bubbly liquids found in food applications feature non-Newtonian liquid phases whereas most investigations of bubbly liquid rheology have employed Newtonian liquids as the continuous phase. The influence of the nature of the continuous phase on bubbly liquid rheology was investigated using bubbly liquids prepared in the same planetary mixer using a viscous liquid, honey (almost constant shear viscosity with a small elastic contribution) and a shear-thinning liquid (1 wt% aqueous solution of guar gum). The viscosity of the honey was similar to the low-shear-rate limit viscosity of the guar gum solution. Although similar bubble volume fractions (up to 25%) were entrained in each liquid under identical mixing conditions, the bubble diameters in the shear-thinning liquid were about two to three times larger than those in the honey. Introduction of a surfactant into the shear-thinning liquid increased the volume fraction of bubbles to approx. 40% and further increased the size of the bubbles. The presence of the bubbles in the honey caused it to become shear-thinning, to exhibit noticeable elastic effects and exert significant normal stress differences. The honey-based bubbly liquids exhibited many of the features in the simulations of Loewenberg and Hinch (1996) and fitted the model of Llewellin et al. (2002) well. In the guar gum solution, these characteristics, already present, were accentuated strongly by the presence of the bubbles. Subjecting both types of bubbly liquids to high shear rates caused the volume fraction of bubbles to decrease and made the bubbly liquids less shear-thinning. Noticeable thixotropy was observed. Shear-thinning, associated with bubble deformation, was observed at lower values of the relative shear stress in the shear-thinning liquid.