Nucleation

Note: It should be observed that this study has been realized on glass contenants; we will later see that this precision has its importance.

 

The birth of the bubble is called nucleation. Two types of nucleation exist.

 

            The homogenous nucleation corresponds to a spontaneous nucleation. Firstly, Laplace's law indicates that a microbubble cannot become a bubble below a certain radius measure.

 

 

           ΔP = 2γ / R                     ΔP in Pa ad R in m

 

• ΔP = P_champagne - P_pocket     therefore    ΔP = 6.0×10⁵ – 1.0×10⁵ = 5.0×10⁵ Pa

• γ the surface tension of the champagne : γ = 5.0 × 10^-2 N.m^-1
• R is the microbubble's radius

 

5.0×10⁵ = (2×5.0×10^-2) / R    or R = (2×5.0×10^-2) / 5.0×10⁵ = 2.0×10^-7 m or 0.20 µm

 

           The radius calculated is the critical radius for which a microbubble becomes a bubble. However, the microbubble notion supposes that its radius is inferior to the critical radius for which a microbubble becomes a bubble, 0.20 µm. The above law indicates that the smaller R gets, the more ΔP increases, so the more the saturation increases. The latter being proportional to the mass concentration (Henry's law), this would mean that the mass concentration would also increase, which is impossible. What is more, the Van der Waals interactions, which guarantee the cohesion of the matter, demonstrate that the quantity of energy needed is too high for a microbubble to form without exploding (we would have to break the molecule's bonds). Cédric Voisin, a fluid mechanics expert, measured in his thesis the rate of homogeneous nucleation: it is of 10^-868551, thus almost nil. The homogeneous nucleation is therefore only theoretical because impossible to put into practice.

 

            Nucleation is therefore heterogeneous, which means it has to be provoked. There are three different types of heterogeneous nucleation. The first is « classical nucleation »: during this phase, the bubble forms on a solid body (often the surface of its contenant: glass, crystal...). This nucleation reveals to be impossible according to the Young law (which is too complex to be explained here). The second heterogeneous nucleation is called « pseudo-classical » or « semi-classical » : the bubbles are created thanks to already existing air pockets but which's radius's are inferior the critical one of a microbubble .The last existing nucleation, called « non-classical » nucleation rests on the same principle as the semi-classical nucleation ; the only difference being that in the non-classical nucleation, the air pockets have a greater radius than the bubbles' critical radius. The principle of this phenomenon is simple: air pockes are held back by the liquid (in this case champagne), and these pockets attract CO2.

 

            Scientists have sought a long time the origin of these bubbles' training site. The first reason found was the presence of micro-cracks in the glass. These defects trap the air during the transfer of the champagne from the bottle to the glass. However, the glasses are now made industrially and therefore have virtually no defects. In consequence, the bubbles would be extremely sparse and few if this was the only reason for them to appear. Nonetheless, a glass of champagne that isn't drunk will present about one million bubbles, which is quite a significant number. How could this be explained? Gérard Liger-Belair, a well known scientist, proved that the bubbling phenomenon is mainly due to the presence of cellulose fibers. Their presence can be easily explained by day-to-day phenomena such as wiping these glasses with towels, placing them near textiles or even « contaminating » them with the air which surrounds us and contains a significant amount of fibers. Finally, bubbles can form because of limescale crystals, that can also imprison the air between the liquid and the glass.

 

            The bubble rate due to anfractuosities is very near zero because of the near perfection of the glasses; the limescale crystals represent 5 to 10 % of the bubbles' training sites. Therefore, we are going to concentrate on the cellulose fibers, which account for 90 to 95% of the training sites.

 

           We set up several experiments to show the impact of cellulose fibers on the bubbling. We poured champagne into a sterile glass, and then into a glass gathering normal conditions (which served as a control), and finally in a glass previously wiped with a cloth. Theoretically, we should have gotten a non-effervescent champagne in the sterile glass and a very effervescent champagne in the glass wiped with a cloth.

 

 

           As we can see in the above video, the sterile glass experience did not produce the expected results. However, this experiment was conducted in a sterile room at the laboratory of the University of Reims Champagne and the champagne obtained was still. We can therefore conclude that our experience environment was not sterile enough and affected the experience.

 

 

            While the champagne is being poured, the cellulose fiber traps an air pocket. However, the pressure of this pocket corresponds to the atmospheric pressure of the outside air, that is to say about 1.0 bar. Therefore, the pressure of the pocket is much lower than that of the liquid's. Champagne is over-saturated in CO₂: this gas will want to occupy the air pocket, which is not over-saturated in CO₂. In consequence, the pressure of the pocket is going to increase and, as a result, its volume too. When the pocket reaches its critical radius (0,20 µm), it is ejected from the fiber and begins its ascent. It is noteworthy that, during its' ejection, the air pocket fragments into two main parts: the bubble, which rises to the surface, and a new air pocket, identical to the initial one, which will become the next bubble to be ejected. Mr. Liger-Belair calculated in his studies that the ejection phenomenon lasts on average 4 milliseconds and the bursting phenomenon lasts fewer than 1 millisecond, which corresponds to about 10 bubbles formation in 1 second. We can observe that the bubbling frequency decreases with time, which makes perfect sense because the mass concentration decreases at the same time (This frequency varies between 1 and 30Hertz during the opening).

 

            The cellulose fibers have a radius composed between 5.0 and 10.0 µm. It is therefore much higher than the critical radius. The nucleation that takes place here is therefore the non-classical heterogeneous nucleation.

 

            We now know that the critical radius is inversely proportional to the pressure (Laplace's law); however, Henry's law states that the pressure is proportional to the mass concentration. Therefore, the critical radius is inversely proportional to the mass concentration. When opening a champagne bottle, the mass concentration decreases because of the CO₂ escaping from the liquid. Thus, the critical radius increases with time, which, as a consequence, makes the nucleation of bubbles more and more difficult. Thanks to this explanation, we can understand why after about ten minutes, the bubbles are no longer visible in a glass of champagne.