Experimental System
Experiments were performed in a water tank with a size of 100 mm ×150 mm ×400 mm as shown in Figure 1. The air supplied from two micro-syringe pumps was injected into water through a bent capillary nozzle. The nozzle orifice faced downward and its injection part was inclined at 3±1o from vertical direction. The mass flow rate was monitored by two gas mass flow meters (FMA-A2302 and FMA-A2303; Omega Engineering Inc., USA). The gas injection experiments were repeated for different gas flow rate Q (40-5400 mL/h) and inner nozzle radiusR 0 (0.030, 0.065, 0.085, 0.130, 0.155, 0.205, 0.255 mm). The processes of bubble behaviors were recorded on a high-speed video camera (Mini UX100; Photron, Japan) under illumination by either a LED light (capturing structure of bubble surface) or a mental-halide light (capturing very high-speed coalescence behaviors). The captured high-speed videos were processed with an image processing procedure programmed by MATLAB. The uncertainty of bubble radius is 3% approximately. The detailed information on image processing procedure and uncertainty analysis of bubble parameters has been introduced in our previous work25.