Experiments and modeling of bubbles colliding head‐on in water

Abstract

Experiments and modeling of head-on collision of a pair of bubbles driven by buoyancy in quiescent water are presented. The experiments showed that two bubbles after contacting for a few milliseconds may coalesce or bounce back depending on the collision velocity. As theoretical comparisons, the influence of velocity boundary conditions on the liquid film drainage and the characteristics of two film drainage models with the assumptions of "partially" and fully mobile interfaces under variablevelocity approach condition are studied. The "partially" mobile model is found to be easier to predict rebound case and longer drainage time, and could not obtain the drainage process where the film is thinned to the thickness below 10 −7 m at both low and relatively high Re. As a whole, the fully mobile model with variable-velocity approach shows more reasonable prediction results for the coalescence and rebound of bubbles in the ultrapure water.

Coalescence of two growing bubbles in a Hele–Shaw cell

“…The bubble distortion just before coalescence can also be derived analytically. There have been several theoretical and experimental studies to evaluate the interaction dynamics between drops or bubbles moving to face each other 13 , 14 , 31 , 32 . Among these studies, Lai et al 24 and Chan et al 25 derived an analytical solution of film thickness between two moving drops confined in a Hele–Shaw cell.…”

“…To date, most previous studies on bubble coalescence have focused on the film drainage in a low-viscosity liquid by moving a bubble of constant volume to another bubble or free surface 12 – 14 . The coalescence in these studies is controlled by inertia force and is different from the growth-driven coalescence in a more viscous liquid, such as magma and polymers.…”

On the role of the macroscopic deformation in liquid film drainage between bubbles

“…This suggests that the model predicts an outcome of bubble rebound, which is inconsistent with the experiment. The reason may be that the F Ã add in our previous work 6 is decoupled from the film drainage process. This leads to an earlier decrease in the approach velocity, as shown by the black dashed line in Figure 6C.…”

“…In numerical solution, all spatial derivative terms in the governing equations are discretized using a second‐order central difference scheme. The detailed numerical procedure can be found in Song et al 6 …”

“…31 This may indicate that the deformation of bubbles plays an important role in the collision process. Based on the energy balance of the bubble collision system, Song et al 6 take the additional effect caused by the MacrD of bubbles into account in a force balance model. Then, the bubble coalescence and rebound were successfully predicted by a fully mobile drainage model with variable velocity boundary conditions.…”