why an americano is more likely to spill than a latte

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.