ABSTRACT
The experimental setup consists of a transparent inclined glass plane of width W = 130 mm which is immersed into water (Fig. 1). The tilt angle θ can be adjusted by a fine screwing system. Spherical monodisperse air bubbles are injected from below at the bottom of the inclined plane (the height h of the piling is kept roughly constant in the experiments h 200 mm). Note that the tilt angle is small (θ ranges from 0◦ to 1◦) to ensure that only one layer of bubbles is created in the direction perpendicular to the plane. Bubble size can be controlled by an air pump and is kept roughly constant in the present study [diameter d = (4.0 ± 1.0) mm]. In order to avoid the coalescence of the bubbles, a small amount of surfactant is added
into water. Due to buoyancy, the bubbles rise underneath the inclined plane and tend to pack on the transverse wall placed at the top of the plane.An orifice of width D = (8.6 ± 0.5) mm at the center (x0, y0) of this obstacle allows the bubbles to empty out the silo. Top views of the bubble packing are recorded through the transparent tilted plane by means of a CCD camera. Movies of the evolution of the packing are recorded at a frame rate of 30 frames per second. In order to quantify and to measure the main properties of the flow, each bubble of the packing (approximately 2000 bubbles per frame) has been tracked through image analysis.
