Bubbles and Flow Patterns in Champagne
Is the fizz just for show, or does it add to the taste of sparkling wines?
The Birth of Bubbles
The first step is to elucidate how bubbles themselves come into being. Generally speaking, two methods exist, and sometimes coexist, to generate bubble chains in Champagne glasses. Natural effervescence depends on a random condition: the presence of tiny cellulose fibers deposited from the air or left over after wiping the glass with a towel, which cling to the glass due to electrostatic forces. These fibers are made of closely packed microfibrils, themselves consisting of long polymer chains composed mainly of glucose. Each fiber, about 100 micrometers long, develops an internal gas pocket as the glass is filled. Capillary action tries to pull the fluid inside the micro-channel of the fiber, but if the fiber is completely submerged before it can be filled, it will hold onto its trapped air. Such gas trapping is aided when the fibers are long and thin, and when the liquid has a low surface tension and high viscosity. Champagne has a surface tension about 30 percent less than that of water, and a viscosity about 50 percent higher.
These microfiber gas pockets act as nucleation sites for the formation of bubbles. To aggregate, CO2 has to push through liquid molecules held together by van der Waals forces, which it would not have enough energy to do on its own. The gas pockets lower the energy barrier to bubble formation (as long as they are above a critical size of 2 micrometers in radius, because below that size the gas pressure inside the bubble is too high to permit CO2 to diffuse inside). It should be noted that irregularities in the glass surface itself cannot act as nucleation sites—such imperfections are far too small, unless larger microscratches are purposely made.
Once a bubble grows to a size of 10 to 50 micrometers, it is buoyant enough to detach from the fiber, and another one forms like clockwork; an average of 30 bubbles per second are released from each fiber. The bubbles expand from further diffusion of CO2 into them as they rise, which increases their buoyancy and accelerates their speed of ascent. They usually max out at less than a millimeter in diameter over the course of their one- to five-second travel time up the length of a flute.
Because natural nucleation is very random and not easily controllable, another way to generate bubbles is to use a mechanical process that is perfectly reproducible from one filling to the next. Glassmakers use a laser to engrave artificial nucleation sites at the bottom of the glass; such modified glasses are commonly used by Champagne houses during tastings. To make the effervescence pattern pleasing to the eye, artisans use no fewer than 20 impacts to create a ring shape, which produces a regular column of rising bubbles.