After pouring Guinness beer in a pint glass, a vast number of small bubbles with a mean diameter of 50μm can be observed to descend (top). The rising motion of bubbles creates a clear-fluid (bubble-free) film above the inclined wall (right). The dense clear-fluid film falls, whereas the bubble-rich bulk rises, which is known as the Boycott effect . We can also observe the fascinating texture motion as a number-density distribution of bubbles travelling downwards (bottom left).
To test the effect of inclination angle of the wall on the texture-formation, we poured Guinness beer in an inclined rectangular container, and observed how the texture forms (bottom right). We found the texture-formation is triggered by the inclination angle of the wall. We experimentally identify the critical condition for the texture-formation and conclude that the roll wave instability  is responsible for the texture-formation in a glass of Guinness beer .
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