Continuous formation of microbubbles during partial coalescence of
bubbles from a submerged capillary nozzle
Bubble formation from a downward-pointing capillary nozzle was
investigated in this study. The experiments were conducted at gas flow
rate of 40-5400 mL/h and inner nozzle radius of 0.030-0.255 mm.
Experimental results show that microbubbles were formed continuously at
moderate Weber number, which was not reported in pervious investigations
with injecting gas through an upward-pointing capillary nozzle.
High-speed visualization indicates that the formation of microbubbles
arises from the convergence of the capillary waves induced by the
partial coalescence of larger bubbles. A bubbling regime map is given to
identify the critical conditions for the formation of microbubbles. In
the present air-water experiments, the generated microbubbles are 20-170
μm in diameter. From experimental data, a scaling law for microbubble
size is proposed as a function of Weber and Bond numbers.