Home experiments Observations

Levitating water

V60 from Leyas
Time to look more closely at the surface of your black coffee.

Have you ever sat watching the steam that forms above a hot Americano? Beneath the swirling steam clouds you can occasionally see patterns of a white mist that seem to hover just above the dark brew. Bean Thinking is about taking time to notice what occurs in a coffee cup and yet I admit, I had seen these mists and thought that it was something that was just associated with the evaporation of the water and that “someone”, “somewhere” had probably explained it. So it was entirely right that I was recently taken to task (collectively with others who have observed this phenomenon and taken the same attitude) for this assumption by the authors of this paper who wrote “The phenomenon that we studied here can be observed everyday and should have been noticed by many scientists, yet very few people appear to have imagined such fascinating phenomena happening in a teacup., espresso grind
The water particles in the white mist are a similar size to the smallest particles in an espresso grind. Photo courtesy of, (CC Attribution, No Derivs). The coin shown is a US nickel of diameter 21.21 mm

The authors of the study show that the white mists (these “fascinating phenomena”) are, in fact, layers of water drops that have a typical diameter of around 10 μm (which is roughly the size of the smallest particles in an espresso grind). Although the white mists exist above tea and even hot water as well as coffee, they are probably easiest to see against the black surface of the Americano.

More surprising than the fairly uniform distribution of water droplet size though is the fact that the authors of this study showed that the droplets were levitating above the coffee. Each water droplet was somehow literally hovering above the surface of the coffee at a height of between 10 – 100 μm (which is, coincidentally, roughly the particle size distribution in an espresso grind).

white mists, slow science
You can (just about) see the white mists over the surface of this cup of tea (which is a still from the video below)

One of the questions that the authors of the paper have not yet managed to answer is what is causing this levitation? Could it be the pressure of the hot coffee evaporating that keeps these particles held aloft? This would explain the observation that the mists form patterns similar to those caused by (heat) convection currents. Alternatively perhaps the droplets are charged and are kept away from the coffee by electrostatic repulsion, an explanation that is suggested by the behaviour of the droplets when near a statically charged object (eg. hair comb, balloon, try it). Perhaps the levitation is caused by the droplets spinning and inducing an air cushion under them? Why not design some experiments and try to find out. It would be great if we can drink hot black coffee in the name of science. Let me know the results of your observations in the comments section below. In the meanwhile, here is a video of the white mists in tea, enjoy your coffee:

You can read the study at: Umeki et al., Scientific Reports, 5, 8046, (2015)


Coffee cup science Coffee review Home experiments

Red Door, Greenwich

Red Door Greenwich, Red Door
Interior of Red Door cafe, Greenwich

Red Door in Greenwich is a great escape from the bustle of the busy streets surrounding it. Although it was crowded when we visited, it was still possible to find a table and have a conversation without too much background noise. I had heard good things about Red Door and wasn’t disappointed. Good coffee (from Monmouth), nice cake and warm surroundings. Definitely a place to go to when in Greenwich. The music that was playing was coming from a record player in the corner. A proper turn-table playing vinyl records. Suddenly, there were so many possibilities for stories for a Daily Grind article. There was the fact that records are analogue based (as opposed to the digital CDs), or perhaps I could write about the physics of a valve amplifier and how it relates to the evaporation of water from coffee (some of the physics is very similar). However what I started to get obsessed with is: what would happen if you put a coffee on a record player?

Now, I am an experimentalist and I do have a record player at home but before I could say “what would happen if…” my plans for experimentation with the record player were blasted out of the water. So I had to make a model record player out of a rotating spice rack. This probably worked better as I could control the speed of rotation, though it did make taking photographs tricky.

record player, turntable
The record player at Red Door

So, what would happen if we put a coffee at the centre of a turntable? The movement of fluids in cups and on record players is extraordinarily complex and is indeed very far from my ‘area of expertise’. However, we can start to understand what might be happening in the cup by making some approximations. Our first approximation is that the coffee in the mug rotates as a ‘rigid body’, meaning that it rotates as a whole. As the coffee cup rotates about its central axis on the “record player” the coffee inside the cup will (eventually) also rotate at the same angular velocity (speed of rotation). The fact that there is a rotation means that there is a force acting on the particles in the coffee liquid. This force produces an acceleration that increases with increasing distance from the axis of rotation. Each coffee particle is of course also subject to the vertical action of gravity. The combined acceleration means that each particle is simultaneously being pulled downwards and inwards. As the acceleration due to rotation increases with increasing radius, the horizontal acceleration becomes increasingly dominant away from the centre of the cup. This leads to the familiar curved surface (a dip at the centre of the mug) that we see with rotating fluids.

vortices, turbulence, coffee cup physics, coffee cup science
This polystyrene cup was rotated about its axis before being stopped. The water inside continues to rotate causing turbulent layers at the edges. These have been visualised with a small amount of blue ink.

Yet we know that this cannot be the full story. If we suddenly stop rotating the mug, the coffee in the mug continues to rotate for a while but does not do so indefinitely; it slows down. We can understand this by refining our approximation that the coffee inside the mug rotates as a rigid body. In fact, the coffee is a viscous liquid and the viscosity means that the layer of coffee immediately adjacent to the mug walls will move at the same speed as those walls: Stationary wall, stationary coffee. The coffee towards the centre of the cup meanwhile continues to rotate for a while. Imagine suddenly stopping the record player so that the mug is now still but the coffee inside continues to spin around the central axis. Stress is being produced between the stationary ‘layers’ of coffee next to the mug wall and neighbouring ‘layers’ of rotating coffee. This stress leads to turbulence. We can make this turbulence visible if, instead of coffee we use a mug of water. Rotate the mug of water as before and then suddenly stop the mug rotating. As with the coffee, the water continues to rotate. Now drop a tiny amount of water soluble ink or food colouring into the very edge of the water (I used a cocktail stick dipped in ink and held against the mug wall so that a small amount dripped into the water). As the water continues to spin, the ink is caught up in the turbulence and the vortices it produces can be seen. These concepts of boundary layers and turbulence are important for many applications including weather systems and car design. We need to understand how liquids (or gases) flow past each other in order to predict the weather and we need to know how they flow past solid objects in order to make cars more aerodynamic. In the coffee however I think that this turbulence is one of those things that is worth just creating and appreciating. A great demonstration of beauty, art and science in a mug of coffee.

Please do share your pictures of these coffee cup vortices if you manage to create them, particularly if you are able to see the effect with cream in coffee. You could either write about your results in the comments section below or email me photographs of your coffee and I will include them on this page. As always, enjoy your coffee.

My thanks to Kate & Edward of Red Door for sending me the photos of Red Door.

Extra photos of vortices in a rotating coffee:

Rotating coffee

An attempt at visualising the vortices using cream in coffee. Not so successful though you can see at least 2 well defined vortices in the top left of the image. Introducing the second liquid right at the edge of the mug seems critical, not so easy with cream as it is with ink!

Coffee review Home experiments Observations Science history

Joe’s espresso cafe bar, Victoria

radiant heat, heat loss, heat conduction, infra red, Joe's espresso cafe bar
The slightly ajar door at Joe’s espresso cafe

A few weeks ago I happened to be near Joe’s espresso café bar on the corner of Medway St. and Horseferry Road, with around twenty minutes to spare. Joe’s is an old-style independent café, very focused on their lunch menu and take away coffees. Nonetheless, there is a decent sized seating area in a room adjacent to the ‘bar’ where you can sit with your coffee and watch the world go by on Horseferry Road. It is always nice to come across a friendly café that allows you to sit quietly and people-watch. As I sat and watched the taxis pass by, I became aware of the fact that it had got quite cold. The people who had just left the cafe had left the door to the room slightly open; the cold was ‘getting in‘. Now I know, heat goes out, cold does not come in but sitting there in that café that is not how it felt. Then it struck me, rather than cause me to grumble, the slightly open door should remind me  of the experiments of Carl Wilhelm Scheele (1742-1786).

Scheele was a brilliant chemist but one who performed experiments that would make our university health and safety departments jump up and down spitting blood. Recognised for discovering oxygen in the air (Priestley discovered it a few years later but published first), manganese and chlorine, Scheele also investigated arsenic and cyanide based compounds. It is thought that some of these experiments (he described the taste of cyanide) contributed to his early death in May 1786 at the age of 43. Fortunately, none of this has a connection to Joe’s espresso café. What links Scheele with Joe’s, is Scheele’s discovery of ‘radiant heat’ as he was sitting in front of his stove one day.

Open fire, Carl Wilhelm Scheele, Radiant heat, infra red, convection
Sitting in front of a fire we can observe several different ways that heat moves.

Scheele’s house was presumably very cold in winter. He describes how he could sit in front of his stove with the door slightly ajar and feel its heat directly and yet, as he exhaled, the water vapour in his breath condensed into a cloud in the air. The heat from the stove was evidently heating Scheele, but not the air between Scheele and the stove. He additionally noted that this heat travelled in straight lines, horizontally towards him, as if it were light and without producing the refraction of visible light associated with air movement above a hot stove. Nor was a candle flame, placed between Scheele and the stove, affected by the passage of the heat. Clearly this ‘horizontal’ heat was different from the convective heat above the stove. Scheele called this ‘horizontal form’ of heat, ‘radiant heat’.

A few years later, the astronomer and discoverer of Uranus, William Herschel (1738-1822) was investigating glass-filter materials so that he could better observe the Sun. Using a prism to separate white light into its familiar rainbow spectrum, Herschel measured the temperature of the various parts of the spectrum. Surprisingly, the temperature recorded by the thermometer increased as the thermometer was moved from the violet end to the red end of the spectrum and then kept on rising into the invisible region next to the red. We now recognise Herschel’s observation of infra-red light as responsible for the radiant heat seen by Scheele, though a few more experiments were required at the time before this was confirmed.

sunlight induced chemical reactions, milk
Often milk is now supplied in semi-opaque bottles. Why do you think this is?

Further work by William Hyde Wollaston (1766-1828) and, independently Ritter (1776-1810) & Beckmann not only confirmed Herschel’s infra-red/radiant heat observations but also showed that, at the other end of the spectrum was another invisible ‘light’ that produced chemical reactions. Indeed, milk is often sold in semi-opaque plastic containers because of the fact that the taste and nutritional content of the milk are affected by such sunlight induced chemical reactions.

So, it seems to me that, in addition to an interesting story with which to idle away 20 minutes in a café, this set of thoughts offers a variety of experiments that we could try at home. If we are out, we could try to discern the different ways that heat is transferred from one body to another (as Scheele). If we had a prism, we could perhaps repeat Herschel’s experiment very easily with a cheap (but sensitive) thermocouple and, if we were really ambitious hook it up to a Raspberry Pi so that we could map the temperature as a function of wavelength. Finally, we could investigate how light affects chemical reactions by seeing how milk degrades when stored in the dark, direct sunlight or under different wavelengths. If you do any of these experiments please let me know what you discover in the comments section below. In the meanwhile, take time to enjoy your coffee, perhaps noticing how the hot mug is warming your hands.

Books that you may like to read and that were helpful for this piece:

“From Watt to Clausius”, DSL Cardwell, Heinemann Education Books Ltd, 1971

“On Food and Cooking: The science and lore of the kitchen” H McGee, Unwin Hyman Ltd 1986

Apologies to university H&S departments, you guys do a great job (mostly!) in trying to help to prevent us dying from our own experiments too prematurely.