We study experimentally the gravity-driven bubbly flow inside a large Hele-Shaw cell. The bubbles and foam were created by a series of upside-down overturns of the half-filled cell about its horizontal axis. When the liquid flows down it entraps a large number of bubbles, which remain stable as the liquid contains surfactant molecules. The total number and sizes of these bubbles slowly asymptote to a steady state after dozens of overturns. It takes longer to reach this asymptote when the viscosity of the liquid is larger. The bubbles also become more monodisperse with more cell over-turns. The number and distribution of the bubbles in turn affects the average motion of the liquid phase, which is characterized by the downwards motion of the liquid center of mass. We use high-resolution 6k video-camera to track the trajectories of thousands of bubbles. This required the development of software codes to identify individual bubbles and follow them between video frames. Successful thresholding algorithm required a machine-learning component, which was integrated into the program. This program also needed to account for possible splitting or coalescence of adjacent bubbles. The program can also find the velocities along the trajectories. In this way we can find the vertical velocity of bubbles as a function of their sizes. The smaller bubbles are sometimes observed to move downwards against their buoyancy. This occurs when the viscous stress from the surrounding liquid phase overcomes the upwards buoyancy force. Bubbles with similar sizes were often found to be stacking together and having worm-like rising movement that is faster than their individual rising velocity. The occurrence of the bubble stacking was dependent on the distance between the bubbles, their sizes and their wakes. Clusters of tiny bubbles that are much smaller than the gap of the Hele-Shaw cell were observed to form layers which can severely hinder the overall liquid motion.
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|KAUST Research Repository