The coffee stain. Something we try to wipe away as quickly as possible before it dries, but have you ever stopped to think about why these rings form the way that they do? Pause a moment and think about it. If coffee (and wine) dries this way, why doesn’t the ink from an ink-jet printer dry in the same way when you print out a document? Do you own a smart phone? How do you think that the screen was made? Do you think that solar cells should be used widely as a renewable energy source? Were you aware that, just as with the smart phone screens, many such electronic devices are now printed using electrically conducting inks?
If these inks dried in the way that coffee does, the screens would not work and neither would the solar cells. Indeed, an increasing number of our electronic devices now rely on printing the electronic circuits. What is needed for such printed electronics is that the ink dries uniformly, not with a ring around the edge. It is therefore very important that we understand why coffee dries the way that it does, in order that we can ensure that our conducting ‘inks’ do not dry the same way.
So what causes the coffee ring? Astonishingly this was a question that was not asked until 1997 when it was investigated by a physicist who was contemplating his kitchen work tops, (link here). Perhaps, as Sidney Nagel did, the best way to start to understand the coffee ring would be to make some.
Start by getting a flat plastic plate, some paper, some coffee, soap, red wine if you so like and something with which to place the coffee drops onto the surface of the plate (eg. a Japanese – pointy – chopstick).
- Using the chopstick, form drops onto the surface of the plastic. The more drops you have, the greater the reproducibility-test of the experiment.
- Try forming a similar drop (or drops) on a paper surface.
- Add some soap to the coffee, try to make similar sized drops with the soap.
- Leave these until they dry.
Here is a “before” photo showing a series of drops formed by coffee (Finca La Casa Vista al Valle from Hasbean); soapy coffee and red wine.
After several hours, the coffee drops had formed coffee ring effects:
Although the soapy coffee had spread out into much larger drops than the other two types of drop, the coffee ring was still apparent. What do you think has occurred and why? Would adding more soap have changed anything?
On paper the results were slightly different, this is a “before” photo:
After the coffee had dried, the ‘ring’ effect seemed less noticeable and the coffee was more evenly spread out in the stain. (the large spread of the stain is probably because I diluted the coffee a bit too much in the cafetiere after having drunk most of it).
The cause and consequences of coffee rings
Coffee rings are formed by an interplay of the surface tension effects in the liquid drop and the properties of the surface (whether it is ‘waterproof’ or ‘wettable’). This is why the drops formed onto paper showed different patterns of coffee stain (wettable) than on plastic (waterproof). What about glossy (more waterproof) paper? Try it!
As the coffee dries, the lateral size of the drop remains the same but, as the water evaporates from the droplet, the drop gets flatter. There is therefore a flow process from the top of the drop down towards the edges. As the water in the drop flows downwards it pushes the coffee particles in the drop along with it. These flow towards the edge of the drop and so form coffee rings. For a more in depth explanation click here.
The addition of soap to the coffee lowers the surface tension of the drop and so affects the formation of the coffee ring in two ways. Firstly, the drops start off ‘flatter’; for the same quantity of coffee, the circumference of the drops is bigger. When this is combined with the weaker surface tension of the drop it means that less of the coffee in the drop is carried towards the edge, making the stain more uniform and less ‘ring-like’. As more soap is added, the coffee stains become more uniform circles of coffee. Clearly more soap was needed in the experiments in the photographs for us to properly see this effect. However, this property of soap-like substances to reduce the coffee ring effect is an important survival tactic for the bacterium Pseudomonas aeruginosa (a common cause of middle and outer ear infections, link).
To return to the electronics industry, to avoid the ‘coffee ring effect’ while printing electronic circuits, manufacturers can choose to (i) add a soap (surfactant) to the liquid hosting the ink, (ii) change the surface onto which the circuits are to be printed (wettability) or (iii) change the shape of the molecules forming the ‘ink’ (which affects how the particles are ‘carried’ by the water as the droplet gets squashed).
In this way, electrically conducting inks can be put down uniformly so that circuits can indeed be printed just as we would print normal ink on a piece of paper. On which note, why do you think that we can print ink on paper without coffee ring effects? Perhaps you can do some experiments to find out.