I can understand that the strings of bubbles from the side of a glass of champagne are due to nucleating dirt or other imperfections in the glass surface, but what causes those strings of bubbles in the center of the fluid? They are quite persistent. Are they just dust? — BM, Tehachapi, CA
If there were no impurities or imperfections in a glass of champagne, bubbles would only form through statistical fluctuations—random effects would occasionally bring enough gas molecules together to form (nucleate) a bubble and that bubble would grow and rise to the surface. But such spontaneously nucleated bubbles are extremely rare and form randomly throughout the fluid, rather than in chains of steady bubbles. In fact, bubbles would be so rare in this impurity-free liquid that you would probably not even notice them—the champagne would slowly go flat by losing gas molecules from its surface alone.
In real champagne, chains of bubbles do rise upward from the center of the fluid. These bubbles are clearly forming at suspended impurities. All it takes is a tiny piece of dust to trigger bubble formation. If you swirl the champagne slightly, you should be able to see these suspended chains of bubbles move, indicating that the impurities that are triggering them are also moving with the fluid.