Bubble growth rate from stainless steel substrate and needle nozzles

The formation of bubbles from a substrate nozzle immersed in water is studied experimentally and compared with bubble formation from needle nozzles under the same conditions. Nozzles with different internal diameter sizes in the range of 0.11–0.84 mm and low gas flow rates from 0.015 to 0.85 ml/min are used. The bubble formation is recorded by a high speed video camera and detailed characteristics of bubble formation are obtained. Interestingly, it is realized that the bubble volume expansion rate follows a cyclic behavior for the substrate nozzles while it shows a smooth decrease after an initial increase for the needle nozzles. Force balance analysis suggests that fluctuations in the substrate are related to the rapid increase of the attaching inertial force at the initial stages of bubble formation. In contrast, the inertial force in the case of the needle is smaller and takes longer to acquire its maximum value. The results also show that bubbles emerge earlier on top of the substrate than on the needle but the initial bubble volume is smaller in the former case. However, the bubble expansion rate is larger for the substrate than for the needle and as a result the final bubble volume is similar for both cases. By using experimentally captured values of bubble height and radius of contact line, the Young–Laplace equation predicts well the bubble evolution until just before the bubble detachment. Other bubble characteristics follow similar trends for both inlets with time and volume.