Oral : Dynamics and Coalescence of Parallel Bubbles Rising in Shear-thinning Viscoelastic Fluids: A Lattice Boltzmann Study

A lattice Boltzmann method is proposed to investigate the dynamics of parallel rising bubbles in Giesekus shear-thinning fluids by coupling a central-moment color-gradient model for multiphase flow dynamics with a lattice diffusion-advection scheme for elastic stress evolution. The method demonstrates the capability in handling problems with large density ratios, with its accuracy validated through single bubble rising simulations in both Newtonian and viscoelastic fluids. Systematic investigations are conducted on the effects of dimensionless parameters, including Weissenberg number (Wi), mobility parameter (α), and solvent viscosity ratio (β), on the dynamic behavior of two side-by-side rising bubbles in the Giesekus fluid. Results reveal that the bubbles initially diverge before convergence, with their interaction patterns strongly dependent on these parameters. With increasing Wi, the bubble interaction undergoes transitions from non-coalescence to coalescence and back to non-coalescence, accompanied by an increase in the peak value of separation distance. Higher α maintains consistent peak separation distances while promoting non-coalescence behavior through enhanced shear-thinning effects. Lower β intensifies elastic forces, resulting in larger peak separation distances and accelerated coalescence. Force analysis indicates that Wi and β modulate both lateral and longitudinal elastic forces around the bubbles, whereas α predominantly influences the longitudinal component. The combined effects of Wi and α on bubble coalescence are characterized through Wi–α phase diagrams, revealing a finite coalescence region whose extent is governed by β. This study provides comprehensive insights into the complex interplay between shear-thinning viscoelasticity and two-phase flow dynamics.