The downward movement of the bubble-texture in a glass of Guinness beer is a fascinating fluid flow driven by the buoyant force of a large number of small-diameter bubbles. This texture motion is a frequently observed phenomenon on pub tables. The physical mechanism of the texture-formation has been discussed previously, but inconsistencies exist between these studies. We performed experiments on the bubble distribution in Guinness poured in an inclined container, and observed how the texture forms. We also report the texture-formation in controllable experiments using particle suspensions with precisely specified diameters and volume-concentrations. Our specific measurement methods based on laser-induced-fluorescence provide details of the spatio-temporal profile of the liquid phase velocity. The hydrodynamic condition for the texture-formation is analogous to the critical point of the roll-wave instability in a fluid film, which can be commonly observed in water films sliding downhill on a rainy day. Here, we identify the critical condition for the texture-formation and conclude that the roll-wave instability of the gravity current is responsible for the texture-formation in a glass of Guinness beer.

For a bubble approaching a flat plate, the Marangoni effect on the drainage suppression of a liquid film and the wall repulsive force are numerically studied. We investigated the driving force of gas-liquid interface. We carry out direct numerical simulation using a boundary-fitted grid conforming to the interfacial deformation. Spatiotemporal development of Marangoni stress and liquid film drainage are quantitatively investigated for various Marangoni numbers. The simulated results reveal that the repulsive force on the wall increases with increasing of the Marangoni number, indicating the suppression of the bubble-wall coalescence owing to the surfactant-driven Marangoni effect. For the sufficiently high Marangoni number, the repulsive force reaches at plateau. Such a saturated force is attributed to an excessive addition of the surfactant, which is likely to make the gas-liquid interface non-slip. The inertial force due to the flow in the liquid film and the Marangoni effect are significant for the deformation of the gas-liquid interface to the dimple shape. The dominant driving force of the interface deformation is not the Laplace pressure but the flow pressure.

To clarify the spatial structure of number density distribution of bubbles in stout beer poured into a container, we investigated local time development of the void fraction and velocity of bubbles. The propagation velocity of the texture, i.e. the number density distribution, appearing near the inclined wall of the container is measured by the images analysis. We measured the local void fraction using brightness of images while the velocity of bubbles by means of Particle Tracking Velocimetry. As the result of measurements, we found the local void fraction and the bubbles advection velocity increase and decrease repeatedly with a time delay. We conclude the pattern of the number density distribution of bubbles is composed of fluid blobs which contain less bubbles; extruding and suction flows respectively toward and from the interior of the container form respectively in front and back of the blobs.

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