cold brew

Super cold brew

Cold brew coffee with ice

Cold brew coffee served with ice. Image from

How cold do you drink your cold brew? Poured over ice? As an experimental physicist who works with liquid nitrogen (& helium), I was initially quite intrigued to learn of nitro cold brew coffee. Could it be coffee that somehow uses liquid nitrogen to fast-cool it, what would that do to the taste? You would expect liquid nitrogen (at -196ºC) to rapidly cool the coffee below its freezing point, after all, it is how Heston Blumenthal makes ice cream. To make a drink-able cold-brew with liquid nitrogen would require great skill, especially given the potential health risks. It would be another situation where you may well ask yourself, “what’s the point?”

However, it turned out that the reality was far more mundane, gaseous nitrogen is passed through cold brew coffee to create a drink with a silky mouthfeel. A smooth drink that comes straight from the tap just like stout. Such a drink is going to behave as an ordinary liquid and chilled only to the point that it is kept in the vat. The novelty would presumably come from the mouthfeel introduced by the many tiny bubbles distributed through the drink. Just as with water, if you cooled the nitro-brew below its freezing point it would solidify and form coffee cubes. No real difference to get excited about. But what if there was a very different sort of liquid, a “super liquid”, that didn’t behave like water, coffee or even liquid nitrogen but one that could leak through solid cups?

Superfluid helium is such a liquid. Like water, oil or even liquid nitrogen, when you cool helium (the same gas that is in party balloons)∗, it becomes an ordinary (but very cold) liquid at -269ºC. But unlike those liquids, when you cool it further, below -271ºC, it does something very odd indeed. It becomes a superfluid in which the liquid moves with zero friction or equivalently, zero viscosity (honey is very viscous, water is very much less so).  And it is because of these properties that it can do some astonishing things such as stream through cracks in containers that were thought impermeable (see the video at 0:52m), or even climb the walls of the container it is in (1:13m)!


To explain the behaviour of superfluid helium it is necessary to use quantum mechanics. Indeed, Fritz London (1900-1954) is said to have described both superfluidity and superconductivity (which happens in solids) as “quantum mechanisms on a macroscopic scale”. At the heart of the theory of superfluidity is the idea that the helium atoms fall into the lowest energy ground state possible, a Bose-Einstein condensate. To form a Bose-Einstein condensate, the particles (atoms of helium) have to  be bosons rather than fermions. All particles in nature can be categorised as either bosons or fermions. The difference between the two types comes from another quantum property of particles, the spin. Spin is related to the angular momentum of the particles and, this being quantum mechanics, can take only discrete values, either whole number or half integer numbers.

cold brew, doublemacbex

Another photo of cold brew coffee, this time from Bex Walton (flickr) – note the condensation around the rim, much could be said about that. Image CC licensed.

Bosons are particles with integer values for spin, fermions are particles with half integer values. Most of the elementary particles you will have heard of are fermions: electrons, protons, neutrons, they’re all fermions. Some particles however, such as the photon (the particle of light) are bosons. Helium 4 atoms are effectively composite bosons, because of the combination of 2 protons, 2 neutrons and 2 electrons that make up the atom. When you add their individual (half-integer) spins, you will get an integer spin, hence a boson not a fermion. The distinction is important because while bosons can share a lowest energy state (the Bose-Einstein condensate), fermions cannot. Quantum mechanically, no two identical fermions can share an energy level (the Pauli exclusion principle), so you can never get to a state where all the fermions are in the lowest energy state. There are practical, every day consequences of this for us, such as the way metals such as copper conduct electricity and heat, the fact that the electrons in the metal are fermions turns out to be crucial for us to understand how metals ‘work’. In contrast, the fact that the helium atoms are in the lowest energy state in super-fluid helium means that the ‘liquid’ behaves very strangely indeed.

We seem to have come a long way from the idea of a cold coffee. But perhaps next time, if someone offers you a “super cold brew” take a moment to think of the physicists who get to play with some real super cold superfluids†. Hope you enjoy the video.


*Technically it is Helium 4 that becomes superfluid at 2.2 K (-271ºC). The rarer isotope, Helium 3, does not become superfluid until much lower temperatures and even then, the superfluidity has some very special properties.

†Although I do get to work with liquid helium (and although it is mostly helium 4), I work at the relatively ‘hot’ temperatures at about -269C. At this temperature the interest is not so much in the liquid helium itself but its use as a coolant for other materials.



A link between high blood pressure and drinking cold brew through a straw

Straws with viscous liquid (milkshake) in them

Drinking milkshake through a straw or two.

How do you drink your cold-brew? How about iced-coffee or iced-tea? Would you drink it through a straw? Maybe a smoothie or a milkshake would be ok. Perhaps you’ve noticed that you need a large straw to drink that milkshake while a small straw works for ‘thinner’ drinks. But what is the connection between this and the measurement of your blood pressure? It is not that drinking coffee gives you high blood pressure or the reverse. That question can be left for discussion on other websites. No, the question is, can drinking a milkshake through a straw give you an insight into the problems of high blood pressure caused by the build up of cholesterol?

If you are currently in a café, why not try an experiment. Get two straws and try drinking a cold drink using both of them together. It’s tricky but it is do-able, you can drink your drink. Now place one straw such that it is ‘sucking’ on the air outside the glass with the other straw still in the drink. Without cheating you can no longer ‘suck’ up that cold brew. Plugging either end of the ‘free to air’ straw enables you once again to drink your coffee. This experiment demonstrates that you are not really ‘sucking’ the liquid through the straw, rather you are generating a pressure difference between the top of the straw (a lower pressure in your mouth) and atmospheric pressure (higher pressure, around the drink) that pushes the liquid through the straw. Attempting to drink through two straws when one is open to the atmosphere cancels out that pressure difference.

2 straws

The straw on the left has a diameter of 3mm, on the right, 6mm.

Now another experiment. How do straws of different diameters affect the amount of liquid you can ‘pull’ through the straw? Try it. I have two straws in this picture, the smaller one has a diameter of 3mm, the larger one a diameter of 6mm. It takes a lot longer to drink a quantity of liquid through the smaller straw than it does the larger straw (assuming that you are drinking the same drink with each straw). For example, I drank 200ml of water in 10-12 sec with the larger straw but 26 sec with the smaller straw.

Back in the early nineteenth century two people were each investigating how liquid flowed through narrow tubes. Jean Leonard Marie Poiseuille (1797-1869) was investigating tubes of diameter 0.013-0.65mm in order to understand the flow of blood through capillaries in the body. Gotthilf Heinrich Ludwig Hagen (1797-1884) was investigating tubes between 2.3-6mm diameter (the same as the straws in the picture). Although they came to their conclusions independently, their work now forms the basis of parts of our understanding of the circulation of blood in the body†. What is now known as the Hagen-Poiseuille law states that the flow of liquid through the straw (or blood vessel) is proportional to the pressure difference between the two ends of the straw (how much you ‘suck’ so to speak) and the radius of the straw raised to the fourth power*. That is, it is the radius x radius x radius x radius. Doubling the radius of the straw results in a 2x2x2x2 (= 16) increase in flow rate.

Experimenting with the two straws does not give you quite the 16x difference that you may expect from this law perhaps partly because the flow into the straw is turbulent. If you maintained a uniform flow through the straws, you should find that the difference in flow rate between the two straws would be closer to 16x.

straw, water, glass, refraction

A straw in water, another physics-phenomenon that is worth contemplating for a while.

Of course, what applies to straws applies equally well to arteries or even the alveoli in your lungs. If your arteries get clogged by too much cholesterol, the reduction in the diameter of the artery leads to a reduced flow of the blood. A decrease in the diameter of an artery by just twenty percent more than halves the flow rate of blood through it (thereby increasing the blood pressure required to maintain ‘normal’ flow rate). Similarly the constriction of the alveoli in the lungs of asthmatics reduces the flow rate of air through the lungs in an asthma attack.

So it is not quite the fact that drinking cold-brew through a thin straw can give you high blood pressure. It is rather that thinking about how liquid moves through straws can help you to think about what is going on in your body. Those arteries of yours may be worth thinking about as you sip your cold brew this summer, whether or not you do so through a straw.


*The Hagen-Poiseuille law states that the flow rate F = ΔP.(r²)²/(8ηl) where ΔP is the pressure difference, r the radius of the straw, η is the viscosity of the liquid and l the length of the straw (or artery). Perhaps you can see why you will need a larger diameter straw to drink a milkshake.

†Blood Pressure Measurement, An Illustrated History by NH. Naqvi and MD Blaufox, Parthenon Publishing (1998)