As may be apparent from my earlier posts, I’m interested in “dangerous” foods and drink and playing with fire. This week, I’ve come across this flaming cocktail idea called “The Backdraft.” Here’s a bloke from New Zealand or Australia (I think) drinking one.
The recipe, abridged from Wikipedia is as follows
- A saucer is placed on a counter or table.
- A shot glass is placed in the center of the saucer, filled with Sambuca
- A pint glass is filled with 1 – 2 shots of Cointreau. Swirl this in the glass to coat the sides
- The Cointreau is lit and allowed to burn until the sides of the glass become warm to the touch
- The lit Cointreau is poured into the shot glass, igniting the Sambuca.
- The pint glass is lowered over the flaming liquid. As the atmosphere cools inside the pint glass it will try to suck the alcohol on the outside back into the upside down pint glass. This backdraft effect is the origin of the drink’s name.
- The glass is removed and a straw is used to suck up the alcohol from the saucer and shot glass.
That seems a relatively straightforward preparation of a drink only a bit less goofy than a Flaming Moe. But I’m trying to understand the physics of the Backdraft. What causes the vacuum in the upside down pint glass? The Wikipedia explanation follows, but I’m not sure it’s correct.
Backdraft physics follows
Once a gas has been warmed up, it tends to expand to fill a volume. It will replace other gases and expand due to its heat content. If this is done in an enclosed sealed space, and then the heat is taken away, as the gases cool, they condense, and decrease in volume, and create a vacuum. Thus when the flaming alcohol in a backdraft is covered with a pint glass over a saucer, the air (a heavier gas) is replaced with warm alcohol vapour (lighter gas)and warm air. As the remaining oxygen is used up, the fire in the pint glass goes out, and the heat source goes away. The alcohol vapor/air mixture now in the glass cools and begins to create a vacuum. This vacuum is responsible for sucking any liquid at the outside bottom of the pint glass further inside (as the seal of the glass and the saucer is not perfect). Once the majority of the liquid is inside the upside down pint glass, sometimes further oxygen can be seen to bubble up into the glass. At some point an equilibrium will occur, where the internal vacuum will hold the liquid inside the glass. This can be great enough at times, that the glass can be lifted, and the saucer will remain stuck to its underside.
When the pint glass is removed, ice is immediately added, thus causing the condensation of the alcohol vapour, creating a white mist in the glass. By covering the glass with the hand, this vapour is trapped until it is extracted by the process of inhalation, usually through a straw.