americano

From a latte to a mangrove swamp

Latte art scutoid tulip
Pretty to look at as well as preventing coffee spill. What is there not to like about latte art?

A few years ago a study revealed why you were more likely to spill an Americano than a latte. It was found that a layer of bubbles on top of a liquid could reduce the amplitude of any ‘sloshing’ produced as you walked with the cup. As the latte has more bubbles than an Americano (or long black), the Americano would slosh more and so spill more easily: if you want to grab a take-away coffee, either grab a lid or order a cappuccino.

It seemed that the bubbles were reducing the amplitude of the slosh because they were causing friction at the sides of the vessel holding the liquid (we would probably say the coffee cup). This friction reduced the energy of the vibration and so decreased the amplitude of the slosh. Without any bubbles, the researchers had produced a ‘slosh’ with an amplitude of 1cm (in their vessel which was about 7cm across). As they added layers of bubbles, this amplitude decreased until at bubble layer thicknesses of five bubbles and more, the amplitude of the slosh was 0.1cm. Bubbles reduced the amplitude of the slosh by a factor of ten.

A few years on and a different set of researchers wondered about the implications of this research on the break-up of ice sheets. The concern was that as the Earth’s ice melted, winds could generate larger amplitude waves on the (now liquid) water surface which, as they impacted the remaining ice sheets could cause them to fracture and crack, thereby accelerating the rate of ice-loss in the polar regions. And yet there is a question. If the ice cracks up and starts floating as icebergs on the water’s surface, could this affect the amplitude of the waves generated by the wind? Could the floating icebergs act similarly to the bubbles of the latte in the earlier study?

Personally I prefer drinking a pour over coffee without milk and in a proper cup. But if you are going to take-away, maybe you need to order a cappuccino instead.

Now of course, there are no container walls in the sea but there are plenty of other mechanisms by which a layer of floating objects may reduce the amplitude of a vibration. In particular, if there are a group of floating objects on the surface of a body of water, as the wave moves up and down the objects move up and down with it, but they also move horizontally. As they move horizontally, away from and then towards each other, there has to be a localised liquid flow into and out of the space between the particles and this offers a way of transferring energy from the amplitude of the wave into a different water movement. This effect increases as more layers of floating objects are added to the water, just as with the latte study. The reduction in the wave height is dependent on the thickness of this layer and, surprisingly, not on the size of the floating objects themselves.

Thinking about these results can help us to understand how mangrove swamps help to protect the coastline during storm surges. During the 2004 tsunami, it was shown that villages behind mangrove swamps in a certain region of India suffered less damage during the surge than villages on unprotected areas of coastline. The mangrove swamps were reducing the height and energy of the surge to make it less destructive. What was it about the mangroves that acted as a coastal defence? Studies since 2005 have emphasised the importance of the aerial root structure of different species of mangrove tree, as well as the density and height (age) of the trees. As the water surges past these roots or branches, they are moving and causing friction for the incoming water, causing localised water flows and removing the energy from the incoming wave. In a sense they are reducing the amplitude of the incoming wave in a way we can understand by contemplating our sloshing latte. This has obvious implications for coastal defence and accordingly authorities around the world have been planting mangrove swamps to protect coastal areas.

Thames, Canary Wharf
The shore of the Thames at low tide. How does the coastline affect the wave dynamics of our water ways? What concentration of plastic bottles and littered take-away cups would result in an alteration of the wave dynamics at the shore line?

These recent efforts for replanting mangrove swamps though come with a history of a 35% reduction worldwide in the area of mangrove swamps between 1980 and 2000. This becomes a further problem because the mangrove swamps have been shown to be excellent carbon sinks, offering a way to reduce atmospheric carbon dioxide and trap it within biomass. A possible sign of hope however is that the existing effects of climate change are causing a growth in the area of coastal mangroves as salt marsh gives way to mangrove in latitudes that have previously been too cold for the mangrove trees to survive the winter. This growth in mangrove swamp offers both a level of coastal protection and a possible negative feedback mechanism for the effects of climate change, though it is unclear what the effects would be of the changing eco-system on the diversity of life in the coastal regions.

There is perhaps one last point to notice before we finish our coffee. There are regions of the ocean that now contain hundreds of square kilometres of floating plastic waste. Even close to our own shorelines and in our river network, plastic waste litters the water. What effect (if any) are these having on the wave dynamics at sea and in our rivers? One more thing to ponder as we carefully walk along sipping our take-away.

Coffee Damping

vortices in coffee

Vortices behind a tea spoon

How often do you allow yourself to get bored? Or to sit in a cafe and take your time to enjoy your coffee properly, noticing its appearance, the smell ‘landscape’ of the cafe, pausing while you absorb the sounds of the cafe and playing with the feel of the coffee while you create vortices with your tea spoon?

If you regularly drink black coffee, you may have noticed how these vortices form more easily in the coffee once the crema has dispersed. Intuitively this may seem obvious to you, perhaps you wouldn’t even bother trying to form these vortices in a cappuccino, you’d know that they wouldn’t appear. The bubbles of the crema (or the milk in the cappuccino) quickly kill any vortex that forms behind the tea spoon (we’d technically call it ‘damping’ them). But even when we are aware of this, it is still surprising just how quickly the crema stops those vortices. Try forming a couple of vortices in a region of black coffee close to a region of crema. Indeed I thoroughly recommend ordering a good black coffee in a great cafe somewhere and just sitting playing with these vortices all the while noticing how their behaviour changes as the crema disperses.

latte art, flat white art

Latte art at The Corner One. Lovely to look at but not good for the vortices.

The damping caused by bubbles on the surface of a coffee is responsible for another phenomenon that you may have encountered in a cafe but, to be fair, are more likely to have noticed in a pub. Have you ever noticed that you are less likely to spill your cappuccino between the bar and your seat than you are your lovingly prepared filter coffee? Or perhaps, in the pub, you can get your pint of Guinness back to the table more easily than your cup of tea? (At least for the first pint of Guinness)

Back in 2014, a team investigated the damping properties of foam by controlling the size and number of bubbles on top of a liquid as it was vibrated (sloshed) about. They found that just five layers of bubbles on top of the liquid was enough to significantly damp the liquid movement as it vibrated from side to side. That is, five layers of bubbles suppressed the sloshing (try saying that after a couple of pints of Guinness). Much as I dislike emphasising the utility of a piece of science, this work has obvious implications for any application that requires the transportation of liquids such as the transport of oil containers. There is an obvious need to suppress the effect of liquid oil sloshing from side to side as it is transported by boat for example.

The foam on our latte or crema on our long black should indeed give us pause for thought as we sit in a cafe enjoying our coffee.

 

 

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.

ineedcoffee.com, espresso grind

The water particles in the white mist are a similar size to the smallest particles in an espresso grind. Photo courtesy of ineedcoffee.com, (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)