FEATURE ARTICLE
Bubbles and Flow Patterns in Champagne
Is the fizz just for show, or does it add to the taste of sparkling wines?
Guillaume Polidori, Philippe Jeandet, Gérard Liger-Belair
Fizz and Flow
The displacement of an object in a quiescent fluid induces the motion of fluid layers in its vicinity. Champagne bubbles are no exception to this rule, acting like objects in motion, no matter whether the method used to produce them was random or manufactured. Viscous effects make the lower part of a bubble a low-pressure area, which attracts fluid molecules around it and drags some fluid to the top surface, although the bubbles move about 10 times faster than the fluid.
Consequently, bubbles and their neighboring liquid move as concurrent upward flows along the center line of the glass. Because the bubble generation from nucleation sites is continuous, and because a glass of Champagne is a confined vessel, this constant upward ascent of the fluid ineluctably induces a rotational flow as well.
To get a precise idea of the role bubbles play in the fluid motion, we observed a Champagne flute with single nucleation site at the bottom. A bubble’s geometric evolution is well studied in carbonated beverages. For example, we know that the bubble growth rate during vertical ascent reliably leads to an average diameter of about 500 micrometers for a 10-centimeter migration length in a flute. In fact, for such a liquid supersaturated with dissolved CO2 gas molecules, empirical relationships reveal the bubble diameter to be proportional to the cube root of the vertical displacement.
Another property of bubbles is that they can act as either rigid or flexible spheres as they rise, depending on the content of the fluid they are in, and rigid spheres experience more drag than flexible ones. Champagne bubbles do not act as rigid spheres, whereas bubbles in other fizzy fluids, such as beer, do. Beer contains a lot of proteins, which coat the outside of the bubbles as they ascend, preventing their deformation. Beer is also less carbonated than Champagne, so bubbles in it do not grow as quickly, making it easier for proteins to completely encircle them. But Champagne is a relatively low-protein fluid, so there are fewer surfactants to stick to the bubbles and slow them down as they ascend. In addition, Champagne’s high carbonation makes bubbles grow rapidly on their upwards trip, creating ever more untainted surface area, in effect cleaning themselves of surfactants faster than new molecules can fill in the space. However, some surfactants are necessary to keep bubbles in linear streams—with none, fluid flows would jostle the bubbles out of their orderly lines.
We carried out filling experiments at room temperature to avoid condensation on the glass surface, and allowed the filled glass to settle for a minute or so before taking measurements. Our visualization is based on a laser tomography technique, where a laser sheet 2 millimeters wide crosses the center line of the flute, imaging just this two-dimensional section of the glass using long-exposure photography. We seeded the Champagne with Rilsan particles as tracers of fluid motion. These polymer particles are quasi-spherical in shape, with diameters ranging from 75 to 150 micrometers, and have a density (1.060) close to that of Champagne (0.998). The particles are neutrally buoyant and do not affect bubble production, but they are very reflective of laser light. It is amazing to see the amount of fluid that can be set in motion by viscous effects. In our resulting images, a white central line corresponds to the bubble train path during the exposure time of the camera, and the fluid motion is characterized by a swirling vortex that is symmetrical on both sides of the bubble chain. We were able to reveal the same vertical structures with fluorescent dye.
The vortex-pair in the planar view of our image can be extrapolated to show a three-dimensional annular flow around the center line of bubbles. This means that a single fixed nuclear site on the glass surface can set the entire surrounding fluid into a small-scale ring vortex. But what really happens in normal Champagne-tasting conditions, with multiple nucleation sites? Is the entire volume of the Champagne affected? Are there different mixing flow patterns according to the method of effervescence? To answer these questions, we investigated two cases: one where only random nucleation sites are present and another where only controlled effervescence occurs.
» Post Comment