INLINE COALESCENCE OF TWO BUBBLES ASCENDING IN A SHEAR-THINNING VISCOELASTIC LIQUID

Direct numerical simulations (DNS) are performed to investigate the inline rise of a pair of three-dimensional (3D) air bubbles in a viscoelastic liquid using the volume-of-fluid (VOF) approach with an adaptive mesh refinement technique. The exponential Phan-Thien Tanner (PTT) model is employed as the non-linear viscoelastic constitutive equation for the liquid. The numerical model has been validated by comparisons with previously published results, including the terminal velocity jump discontinuity of an isolated bubble rising in a viscoelastic fluid. Focusing on the inline rising bubble pair in such a viscoelastic medium, we found that for pairs including a leading bubble and a trailing bubble, drafting-kissing scenario occurs before the bubble-bubble coalescence. The long-range repulsion and the short-range attraction due to fluid elasticity are critical to the bubble pair interactions. Interestingly, the squeezing flow near the growing bubble neck seems to delay the coalescence process. The capillary wave propagating down to the coalesced bubble tip together with the extensional flow behind the stretched bubble, determines the generation of satellite microdroplets along the tail of the coalesced bubble.