Bubble cascade in Guinness beer is caused by gravity current instability

Tomoaki Watamura,Fumiya Iwatsubo,Kazuyasu Sugiyama,Takashi Shiono,Yuko Yotsumoto,Kenichiro Yamamoto

doi:10.1038/s41598-019-42094-0

Abstract

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.

Highlights

Following Archimedes’ principle, bubbles in liquid generally rise because of the gas-liquid density difference
The reduction in the rise velocity of bubbles in large bubble-volume-concentration fluids, known as the hindered velocity[9,10], was suggested to be one of the mechanisms, i.e., the velocity difference in the bubble swarm is involved in the downward movement of the bubble swarm as waves, even when the buoyant bubbles themselves rise[4]
The presence of a clear-fluid film was not considered in the model[5], and the hydrodynamic condition for the texture-formation have not been discussed so far

Summary

Introduction

Following Archimedes’ principle, bubbles in liquid generally rise because of the gas-liquid density difference. The computational investigation has concluded that when Guinness is poured into a typical pint glass, which widens towards its top, the rising motion of bubbles creates a clear-fluid (bubble-free) film above the inclined wall. We can observe the descending bubbles entrained into the downward flow in Guinness, which is seemingly paradoxical in light of Archimedes’ principle. This physical explanation leads to the understanding of the descending motion of bubbles, the mechanism underlying the texture-formation still remains an open problem. We propose a falling liquid film model assuming stratified two-fluids, and we experimentally quantify the critical condition for the roll-wave formation involved with the hydrodynamic instability in the gravity current in the inclined wall vicinity

Methods

Results

Conclusion