Texture-formation and motion of bubbles. (a) Snapshots of the bubble-concentration wave in the rectangular container for various inclination angles β. (b) Microscope image of bubbles in the trapezium container (left), and a superimposed display of bubbles extracted from the image by the template matching method28, showing the bubble-velocity vector obtained via the particle tracking method29 (right). Time-sequential images were captured by a high-speed video-camera mounted on a microscope. The corresponding movie is included as Supplementary Movie S2. (c) Temporal variation in the brightness of image B (top), the vertical vx (middle) and horizontal vz (bottom) bubble velocities. The shaded regions correspond to envelopes given by the local standard deviation of the bubble velocity in the measurement area.