Guinness bubbles FAQ
- Why is this effect important? The effects are important because they tell us about how liquids flow, for example, in industrial processes and manufacture. Liquid flows are important in a wide range of circumstances ranging from medicine to oceanography. Drinks and food manufacturers also spend a considerable amount of effort and money to make their products appealing: easy pouring and a nice head is one of the aspects that we consumers look for in our beer.
- Is the shape of the glass important? The shape of the glass is not that important. The best results are obtained in glasses that get wider at the top. The pattern of flow will be different in the different shaped glasses, but not to the extent that would remove the effect completely.
- Is the type of gas important? The fizz in most lagers, beers and soda is carbon dioxide, which is more easily dissolved into the liquid. This is why you can see streamers of bubbles appear to form out of nowhere at the side of a glass of beer or cola: the dissolved carbon dioxide undissolves to form bubbles at tiny defects in the glass surface: they continue to grow, ingesting more dissolved carbon dioxide as they go up (as shown, left, for cola). In Guinness, the gas is nitrogen (which makes up roughly three quarters of the air we breathe). Nitrogen does not dissolve as well in liquid as carbon dioxide, and so the bubbles do not grow like in lager or cola.
- Method of pouring? The way the Guinness is poured is not crucial. Despite the angle/position of pouring, it is the upward flow of bubbles in the center of the glass that causes the circulation. If you pour it so that there are hardly any bubbles produced, then you will not see the effect.
- What about viscosity? More viscous liquids flow more slowly (e.g., syrup). The viscosity of Guinness is slightly different than that of water, but not enough to affect the movement of the bubbles that can be seen.
- Guinness in the lab? You might wonder how we got a good supply of Guinness in the lab. The experiments took many days, and it would have been difficult to set up all the equipment in the local bar! Fortunately, the Guinness company makes their draught stout available in cans, using a special device (the "widget") that creates all those creamy bubbles. Although we tried refrigeration before using them, condensation on the glass caused difficulties, so we used the cans at room temperature.
- What happened to the Guinness you used? Alas, we poured it away. Don't be sad - you might never have seen any of the pictures if we had actually drunk any of it during filming!
- What's a widget? The widget is the patented device (also called a "smoothifier") in the special draught-flow can or bottle that contains a small amount of Guinness and nitrogen gas at very high pressure. If you shake one of the cans you can hear it rattle. Why not open a can up and have a look? When the can/bottle is opened, the gas is suddenly released through one (or more) pin-sized holes. This creates a lot of little bubbles.
- Do simulations and experiments agree? Our experiments and conclusions agree very well with the simulations carried out by Nurul Hasan and Clive Fletcher. The sinking bubbles in the experiments seem to be slightly larger than in the simulations. The experiments also suggest that the bubbles inside the glass (the ones that float up) seem to be nearer to the walls in the experiments. An interesting detailed comparison would come from looking just below the head, where we see almost equal numbers of bubbles going up and down.