The surface tension T is a function of the type of chocolate and its viscosity, so this means that for a given chocolate the smaller bubble will have a higher pressure inside them than the larger ones, for the same outside pressure. If two bubbles come together, then the pressure will move from the high-pressure bubble to the low-pressure one, with the result that the large ones will grow at the expense of the small ones.
Very importantly, if the difference in pressure (p2-p1) increases, then so must the radius of the bubble in order to maintain the balance of eqn 9.2. This means that if you take a liquid chocolate under atmospherics pressure and then place it under vacuum, any small bubbles will grow dramatically, provided the chocolate is thin enough to move. In addition, more air will be released from the fat phase, creating new bubbles and enlarging others. This is because the solubility of a gas in fat increases with pressure. If the gas had been in the saturated condition at the original pressure, then the more the pressure is reduced, the more gas is released.
Changing the difference in pressure can be done In two ways. In one, the liquid chocolate is put in a mould and then placed in a cold vacuum box. The air bubbles form very rapidly and then the chocolate sets, preventing it from collapsing again. Sometimes additional gas, usually carbon dioxide, is mixed into the chocolate before the vacuum is applied.
In the second method, the gas is vigorously mixed under high pressure with the liquid chocolate. This dissolves the gas and forms micro-bubbles, both of which form the larger bubbles in the product when it is deposited into the chocolate shell and returns to normal atmospheric pressure. The chocolate is then cooled so that the bubbles are preserved.