One of my favorite activities is to order a Boddingtons, a creamy British ale. As good as it tastes, I always anticipate the visual splendor that is the head. As it forms, tiny bubbles flow downwards. If you're used to beers and sparkling wines where the bubbles go up, it's entrancing.
Boddingtons and Guinness are two examples of a beer that foams because there is both carbon dioxide and nitrogen dissolved in the beer. The CO2 results from the fermentation process, but the nitrogen must be introduced artificially. In a bar where these beers are served on tap, nitrogen is pumped into the beer at high pressure. In a can or bottle, nitrogen has already been added, but a widget is used to introduce pressurized nitrogen to initialize the bubbling process.
Have you ever wondered why bubbles form at all? In order for a bubble to form there must a small gas pocket. Gas then diffuses into the pocket, and when it reaches some critical size, it detaches and - voila! A bubble. I have always thought that the bubbles formed on the surface of the glass where small imperfections created the gas pockets. I remember my father taught me an important lesson - always use the same glass. He thought that in a freshly washed glass there would be enough soap (surfactant for you science-y folk) left behind to coat the sides of the glass and inhibit bubble formation.
Then in 2002, French scientists studying champagne (of course) found that bubbles form on small cellulose fibers. These fibers are thought to be added either by the cloth used to dry the glass or by falling from the air.
Two Irish mathematicians - who I'm sure enjoy a pint of Guinness now and then - have shown that cellulose is an efficient method to promote bubbling in these 'nitrogen' beers and have proposed that the widget could be replaced by a coating of cellulose on the side of a bottle or can.
Please enjoy your beer responsibly and scientifically.