Gravity wave in a glass of Guinness beer

To address an open problem how the bubble texture forms in a glass of Guinness beer, we investigated the propagation velocity of texture and individual bubble velocity by means of high speed microscope imaging. We inferred the liquid phase velocity from the observation of bubbles motion. As the result of measurements, we found that the texture formation at the Froude number, defined by the liquid phase velocity and the bubble-free layer thickness, Fr > 1 is excellent accord with the onset of primary instability of the falling liquid film. Our experimental observation suggests that the gravity wave of the stratified flow is responsible to the texture-formation of Guinness beer.