Rheology of bubbly liquids containing a non-Newtonian continuous phase
Bubbly liquids are two-phase materials where gas is dispersed in a liquid with bubble volume fractions ranging between 0 and 0.5. Bubbly liquids are found in many real-world contexts and their bulk rheological behaviour is determined by the continuous phase rheology, the bubble phase properties and the interaction between the two phases. The majority of research concerning bubbly liquids has focussed on materials with a Newtonian continuous phase. However, many of the bubbly liquids found in industrial and consumer settings have a viscoplastic continuous phase. This project has investigated the rheology of bubbly liquids with a viscoplastic continuous phase, namely a commercial polymer solution called ClearGlideTM that belongs to a family of yield stress fluids known as Carbopols. Bubbly liquids with air volume fractions in the range 𝜙a = 0 − 0.5 were produced by whisking the continuous phase in a planetary mixer. Aeration profiles describing how 𝜙a evolved were found to have the same general shape regardless of continuous phase dilution or mixer speed. When plotted using normalised aeration parameters the majority of data sets followed a common curve which was quantified using an exponential relationship. The bubbly liquids had polydisperse log-normal bubble size distributions. The variation in bubble size with continuous phase dilution and mixing time was considered with respect to bubble break-up. Original models accounting for the process geometry were proposed and found to provide a good description of the aeration process. These models provided insight into the mechanisms taking place during whisking, specifically entrainment and disengagement of bubbles. Rheological behaviour was investigated using both shear and oscillatory rheometry. Unaerated ClearGlideTM exhibited typical yield stress fluid behaviour comprising three distinct regions: solid-like linear elastic deformation at low stress, followed by a yielding transition, and subsequent liquid-like shear-thinning at large stress that was described using the Herschel-Bulkley model. Suitable rheological parameters were identified to characterise each region. Bubbly ClearGlideTM exhibited similar behaviour to that of its continuous phase. Aeration generally resulted in a reduction in the rheological parameters with increasing air volume fraction. Correlations quantifying the impact of air volume fraction were determined. The combined effect of the continuous and bubble phase material properties was considered and found to be complex, with the continuous phase having a greater impact on the bulk rheology.
Bubbly liquids, Rheology, Viscoplastic fluids, Yield stress
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