YOU may think that complex equations and alcohol don鈥檛, or perhaps shouldn鈥檛, mix. But make your favourite cocktail and you will unknowingly encounter some of the most complex processes in fluid dynamics, the study of how liquids flow.
When researchers try to predict how a fluid will move, bubble or create waves, they often run into complicated equations. The starting point for solving almost any of these problems is the Navier-Stokes equations, named after Claude-Louis Navier and George Gabriel Stokes. The pair devised them in the 1800s, which also happens to have been the golden age of mixology.
What better way, then, to learn about fluid dynamics than by indulging in some cocktails? Whether it is how foams are made, the formation of unusual clouds or liquids spurting at supersonic speeds, some wonderful surprises can hide in a drink. Roll up your sleeves and dig out your cocktail shaker!
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GIN FIZZ
Experience the miniature marvel of foams
First up, something fizzy. Made from two parts gin, one part lemon juice, a dash of syrup and a splash of soda water, the gin fizz would be simple were it not for its layer of foam.
Foams challenge physicists. At times, they behave like solids; at other times, they act like liquids. Soapy bubbles flow like water when you wash your dishes, but the stiff head of a beer can be sliced off in one.
This difference comes down to the bubbles. When bubbles crowd together, they make a foam. But how foam acts depends on the molecules inside the bubbles鈥� fluid casing. Foamy beers and the frothy milk that tops a cappuccino, for instance, are full of proteins, as is the egg white that traditionally makes the foam in a gin fizz.
Protein molecules are slightly electrically charged and have both water-loving and water-repelling parts. These properties cause them to align in just the right way to keep bubbles from collapsing 鈥� for a while, at least. Stronger attractions between the protein molecules, and a high concentration of them, result in stiffer foams that behave more like solids. Foams with fewer molecules or molecules that barely interact stay runnier.
For a gin fizz, we need our foam to be robust so that it sits on the top of the drink. If you whisk the egg whites vigorously enough, the proteins will stretch out, then link up, leaving you with the perfect pourable foam. If you prefer not to use egg whites, the liquid from a can of chickpeas, known as aquafaba, is a good alternative as it is chock-full of proteins. And if you are struggling to make a foam, adding a bit of cream of tartar will nudge the proteins to stretch more reliably and quickly.
B-52
Create a complex cloud in your glass
The physics, and the flavours, get more complex when you pour a drink with more than two layers. Take the eye-catching B-52. This short cocktail contains equal amounts of three alcoholic beverages: a layer of orangey-gold Grand Marnier sits on top of milky Irish cream, which floats on a deep brown coffee liqueur. And with this cocktail, we can demonstrate something called the Kelvin-Helmholtz instability.
The drink鈥檚 layers stay separate within the glass because each is less dense than the one below. Buoyant forces, caused by the pressure on the molecules at the bottom of each layer being larger than the pressure on the molecules at the top of each layer, counter gravity鈥檚 downward pull. Bartenders pour B-52s one liquid at a time, slowly over the back of a spoon to avoid mixing.
Earth鈥檚 atmosphere and oceans also contain layers of clouds and layers of water of different density, temperature and salinity, respectively. And when two stacked layers of fluid move over each other, be it water or liqueur, spectacularly wispy clouds can arise.
The Kelvin-Helmholtz instability kicks in when the forces of one layer dragging over another overwhelm the buoyant forces that keep them apart. As the buoyant forces start to lose the fight, the top layer of fluid doesn鈥檛 immediately sink into the one below it all at once. Instead, parts of it descend in the form of swoopy, cloud-like structures.
In the atmosphere, the Kelvin-Helmholtz instability causes fluctus clouds that look like breaking ocean waves, but the same physics hides within a glass of B-52. Tilt your glass, or give it a spin, and you might see tiny, wispy clouds appear. But be gentle 鈥� if you use too much force, you will be left with a delicious beige mess.
FRENCH 75
What really happens when champagne pops
The French 75, as well as being tasty, calls for an especially dramatic ingredient. A mix of gin, lemon juice, syrup and some champagne, all the action happens before you pour this drink, in the explosive uncorking of a bottle.
When champagne ferments, yeasts eat up the sugar in grape juice and produce carbon dioxide, giving the drink its fizz. But this also creates pressure inside the champagne bottle meaning that, when you uncork it, jets of carbon dioxide come rushing out.
In 2019, at the University of Rennes 1 in France and his colleagues used a high-speed camera to capture the details of a champagne bottle uncorking. They found that in the first millisecond of the process, the jets achieve supersonic speeds. The fastest they identified went flying at about four times the speed of a Formula 1 car.
The researchers then simulated this process in detail on a computer, which required solving some equations formulated by Navier and Stokes. They found that the gas forms shock waves of various shapes behind the cork, first resembling a crown while stuck behind the slower-moving cork, then forming a foamy cylinder, then finally slowing down and running along the sides of the bottle.
Georges refers to a bottle of champagne as a 鈥渕ini laboratory for the physics of fluids鈥�. But past those first few milliseconds, the only experimentation left for you will lie in deciding whether to swap the syrup in your French 75 for a sugar cube.
RUM PUNCH
Sail a boat on top of your tipple
You can have a lot of fun with a rum punch if you know this secret. But first, let鈥檚 mix one up. The traditional recipe calls for rum and fruit juices mixed with some sugar syrup, grenadine and Angostura bitters. Now you have your drink, you are ready to experience the power of a phenomenon called the Marangoni effect.
The curious result of a difference in surface tension, this effect was amusingly demonstrated in 2013 by at the Massachusetts Institute of Technology and his colleagues. In collaboration with professional chefs, of miniature boats from ingredients like gelatine, agar and melted candies. They 3D-printed silicone moulds, then poured the edible mixture into them to cast tiny vessels with a slit in the back. Then, they set these on rum-powered journeys across cocktail glasses.
Each boat was filled with rum that slowly leaked from the slit at its stern. When rum mixed with the liquid in the glass (which, in the experiment, was water rather than fruit punch), it made the surface tension lower at the boat鈥檚 stern than at its bow. Water molecules started to move from the back to the front of the boat in an effort to even out the tension, propelling it in the process. This is the Marangoni effect.
In the team鈥檚 experiments, the 1.5-centimetre-long boats travelled as fast as 11 centimetres per second. You can try it with fruit juice, effectively preparing your own rum punch as the boat sails. Perhaps there is also an optimum number of cocktails to be consumed before beginning the experiment. Something else to try out over the festive period?
Karmela Padavic-Callaghan is ready聽for the holidays to be-gin
