Robyn Williams: And so to end this Science Show about lateral thinking and the future, more maths. Here's one of our former top scientists, Dr Sophie Calabretto from Macquarie. And please take notes as she speaks.
Sophie Calabretto: Whenever I talk about fluid mechanics, I like to start out with the fact that when we refer to a fluid, we're talking about everything that flows—air, water, blood, mucous, honey, toothpaste, and even room temperature butter—but there are two very important fluids that I often omit from that list, mainly because society gets judgemental when I spruik certain things in the children. Those fluids are gin and vermouth, and with their powers combined they become a delicious martini.
Now, you may be familiar with the cliché but I'm going to propose an alternative way for mixing a martini which uses the raw power of fluid mechanics itself. If I pour any two fluids into a glass, they will naturally mix a little if their chemical properties are similar enough. Luckily gin and vermouth are miscible with each other in any quantity, and when mixed properly will form a homogenous solution. That is, a martini. However, if we wanted to make a cocktail out of oil and water, we'd be in for an immiscible disappointment from the very start. Our chosen fluids would eventually form a homogenous martini when we mix them. If we don't mix them at all, eventually the gin would settle on top because generally the vermouth is denser than gin. And that doth not a martini make.
Now, when I pour any fluid into a container such as a martini glass, the laws of physics tell us that the fluid immediately adjacent to the glass has to move at the same speed as the glass. This is because at a molecular level the fluid tends to adhere to the glass rather than slide over it. For everyone playing at home, this is called the no slip boundary condition and it occurs for all viscous fluids when they encounter a solid boundary. Since our glass is not moving, the fluid next to the glass is not moving either. The fluid in the centre of the glass, on the other hand, can do whatever it wants because there is no solid object cramping its style. What this gives us is a thin layer of fluid in which the velocity rapidly changes from zero next to the glass to the free stream velocity towards the centre of the glass. In fluid mechanics we call this thin layer a boundary layer, and its thickness depends on the viscosity of the fluid. Generally speaking, very viscous fluids like treacle would give way to thicker boundary layers and thin fluids such as alcohol would have thinner boundary layers.
As a fluid mechanistic, I am fascinated by boundary layers. These seemingly innocuous layers of attached fluid can often become unstable, breaking away from the surface of an object and wreaking their stable havoc on the rest of the fluid flow. It is these poorly behaved boundary layers that we use to mix our martinis.
When fluid no longer flows smoothly in nice layers and starts swirling about in a messy chaotic way, we call this turbulence. Turbulence is simply fluid chaos, and since we cannot predict chaos, nor can we predict turbulent fluid motion. What we do know, however, is that turbulence is great at mixing things, and we have some fairly sure-fire ways to ensure that a flow becomes turbulent.
Imagine you have a snow globe. The best way to get the tiny particles of faux snow to mix through the fluid is obviously to shake the globe. When you shake it, what you're actually doing is imparting the system with a whole lot of energy in order to kick it off into a turbulent flow regime. And this is a good way to mix a martini.
Stirring is another good way to harness the mixing power of chaos, but that still requires that you impart a decent amount of energy into the system by use of a spoon or a swizzle stick. But if we can cause our boundary layers to become turbulent, breaking away from the glass and stirring up the contents of our cocktail, we can efficiently mix our martini through little to no effort at all. In fact, even very small disturbances to a boundary layer can be enough to see the growth of instabilities that amplify and eventually lead to a fully turbulent state. Small disturbances could be something as simple as physical imperfections on the inside of your martini glass or even vibrations from the too-loud bass from the speaker system in the cocktail bar as the gin and vermouth are being poured.
So, first turn up the bass really loud, then find the most imperfect glass you can. Then wait, as the wonders of fluid mechanics mix your martini. Or maybe they don't. If you haven't tripped that boundary layer into becoming turbulent, that waiting will soon turn into disappointment, in which case maybe just shake or stir it. So perhaps turbulent boundary layers are not the most foolproof way to mix a martini. In fact, the purpose of my research is necessarily to understand the subtle art of lazy cocktail making, but rather to understand why boundary layers might detach from surfaces and become turbulent. Sometimes turbulence can be desirable, such as when we need to make a cocktail or in large industrial mixing applications. We may want to suppress turbulent boundary layers, such as around aircraft wings, on the hulls of boats, and in blood vessels and pipes. If we don't understand turbulence, we can't use it to our advantage.
So, as I ponder this problem some more, I think I'll have another martini. The usual, thanks, a tequila sunrise. Thank you.
Robyn Williams: Dr Sophie Calabretto, who obviously taught James Bond why his martini should be shaken and not stirred. She was one of our top scientists from Macquarie University.