Characterization of the Performance of an Injector for the Controlled Generation of Microbubbles

We perform a characterization of a recently reported microbubble injector in conditions relevant to microgravity. Injection of bubbles is based on the generation of a slug flow in a capillary T-junction, whose operation is robust to changes in the gravity level. We address questions regarding the performance under different working regimes. In particular, we focus on the regimes found within a large range of gas and liquid injection flow rates. The injection performance is characterized by measuring bubble generation frequency. We propose curves obtained empirically for the behavior of generation frequency and crossover between regimes. INTRODUCTION Bubble generation and management are some of the key issues in the current research on multiphase flows for space applications [1-3]. The absence of the bouyancy force in the microgravity environment makes necessary specific configurations in order to detach bubbles from the injection device. Typical methods used are the coflow [4], in which liquid flows parallel to the gas injection direction, and cross-flow [5], in which liquid flows perpendicularly to the gas, configurations. We propose a configuration [6,7] which is a particular case of the cross-flow one. Two flows of different phases are injected separately into a T-junction (see Fig. 1) with capillaries of the same diameter (1 mm). At the Tjunction, gas bubbles are formed by the action of the liquid cross-flow. In the nominal regime of the bubble generator, capillary forces dominate over buoyancy (small Bond number), and over inertial forces (small Weber number). Bubble formation thus results from the competition between capillary forces and the drag due to the liquid cross-flow (note that the drag can be large even in the small flow limit when the forming bubble occupies the available cross-section of the capillary). This force balance determines bubble size and generation frequency.