Volume 11 (2016) Issue 2 Pages 281-287
A microfluidic device was fabricated to study the collision of bubbles with controlled approaching speed of 0.01-10 mm/s. The collision of bubbles resulted in instant coalescence, delayed coalescence and elastic collision as their approaching speeds gradually decreased. This velocity-dependent coalescence is attributed to the variation of the rotation speed of the bubbles under different approaching speeds, which is observed by adhering fluorescent particles on the bubble surfaces. The theoretical predictions illustrate that the rotation of the bubble helps to reduce the hydrodynamic pressures between two approaching bubbles, which accelerates the drainage of the thin liquid film in between and the coalescence.