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Bubbles behave differently in non-Newtonian fluids (such as paints, gels, mud and clay suspensions) than they do in Newtonian fluids (such as water). Non-Newtonian fluids have a non-linear response to stress. This means that the apparent viscosity a bubble sees varies with the stress along its surface, e.g., at the leading and trailing surfaces. This leads to interesting bubble shapes, as shown for a bubble rising in a Carbopol solution, a typical non-Newtonian fluid used to model others in our laboratory simulations and experiments.

The same happens in flows with many bubbles, as seen by comparing the two images of bubbly flow in a pipe with Newtonian and non-Newtonian liquids. The Newtonian liquid used here was water. The non-Newtonian liquid is a Laponite suspension, another model fluid that is transparent, allowing easy visualization. The bubbles in the air-Laponite flow are larger and elongated due to the non-Newtonian stress response of the liquid.

These differences in bubble behaviour can lead to differences in gas holdup, mass transfer, and flow regime transitions. For Newtonian fluids, these aspects of multiphase flow are often well-described by correlations from literature. However, many fluids encountered in engineering practice like slurries, muds, foods etc. are non-Newtonian, and the same correlations do not apply to them. These fluids require a different toolbox that may include rheological characterization, experiments with model fluids, or simulations that account for non-Newtonian fluid behaviour.