The Influence of Injection Angle on Bubble Formation from a Micro-Pillar

Bubble formation in a microchannel in the presence of a 150 $\mu $m diameter micro-pillar was investigated. Nitrogen stream was injected into water flow in a 225 $\mu $m deep, 1500 $\mu $m wide, and 27.5 mm long horizontal microchannel through 20 $\mu $m slits cut on the micropillar located vertically in the centerline of the channel. Bubble formation in different devices with varying slit angles---with respect to liquid flow---of 0\r{ }, $\pm $30\r{ }, $\pm $80\r{ }, $\pm $110\r{ }, and 180\r{ } were studied for liquid flow rates of 13, 34 and 54 ml/min, and gas flow rates ranging from 0.5 to 7 ml/min. Based on high speed high magnification imaging, three distinct formation modes were observed depending on the slit angle and liquid and gas flow rates: discrete bubbling, attached ligament, and mixed modes. Micro-PIV technique was used to study the liquid flow in vicinity of the pillar to elucidate the phenomena controlling bubble formation.