The influence of thixotropy on bubble growth in thixotropic yield stress fluids: Insights from numerical simulations

Bubble behaviors in structured fluids are of great interests in industrial applications, while there is currently a lack of understanding regarding the effect of thixotropic microstructure on the bubble formation process. To this end, this study explores the influence of thixotropy on bubble growth in thixotropic yield stress fluids by numerical simulations using the Arbitrary Lagrangian-Eulerian (ALE) method. The numerical results reveal that, with the increase in the thixotropy number, the bubbles at detachment transform from inverted conical to spherical shapes at lower gas flow rates, and from spindle to conical shapes at higher gas flow rates, along with the decreased detachment volume and time. It is also found that the effect of gas flow rate varies with different thixotropy numbers. The flow field of the structured fluid reveals that the increases in gas flow rate primarily promote the structural destruction near the bubble tip, while the increase in thixotropy number facilitate the fluid flow around the bubble, with the significant reduction of the low-shear zones and expansion of the yielded zones near the equatorial plane. As a result, modulating the fluid flow with thixotropy number mainly influences the hydrodynamic pressure on the bubble. Based on a force balance model, the forces acting on the bubble are then calculated by integrating the stress on the interface, and it is found that thixotropy number controls the bubble detachment state with the drag effect. Accordingly, the mechanisms governing the influence of thixotropy on drag effect are discussed considering the flow field characteristics and the correlations of drag coefficients. This work helps to deepen the understanding of the bubble behaviors in structured fluids.