I am writing a review at the moment for Annual Reviews in Materials Research on inkjet printing. I have almost finished and I was just downloading a couple of articles on coffee stains when though some morphic resonance related phenomenon, I knocked over my mug of coffee. So an excuse for a blog post on coffee stains.
And just to prove this is really science, here is a link to Deegan’s classic paper on coffee stains as published in Nature (apologies if you don’t have access).
Why is this of interest to anyone? Coffee staining occurs when a liquid containing a concentrated suspension of particles evaporates. If the liquid is a drop then the contact line (where the drop edge is) is where the liquid deposit is thinnest. Evaporation from the drop is driven primarily by local chemical potential and thus the curvature. This means that the first precipitation of the particles occurs at the contact line and prevents the liquid drop from retracting as it evaporates image b below).
If the contact line is pinned, then liquid has to flow from the interior of the drop to replenish that which is evaporating from the drop edge. This flow drags particles (or coffee) with it and increases deposition at the drop edge. In extreme cases practically all the solute or suspension is deposited at the edge of the drop forming a nice (or ugly as beauty is in the eye of the beholder) ring.
Are you still not clear why this is interesting or possibly useful? If we use inkjet printes the image is made up of lots of dots that are deposited as a liquid and then dry to form coloured patches. We can also use this technology to print lines (bar codes) or even electronic circuits. Evaporation from these tiny drops is particularly sensitive to coffee staining (images below).
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So that s why you can find research into coffee staining funded by your favourite funding agencies.
Lovely! (apart from the morphic resonance, to which I am allergic, unlike coffee).
This beats the science of doughnut dunking, every time, in my book.
the solute or suspension is deposited at the edge of the drop forming a nice (or ugly as beauty is in the eye of the beholder) ring
A friend’s father, who is a physicist and an artist, creates fascinating and beautiful images with ink and soap bubbles, using what I suspect are similar phenomena. He varies the inks, the paper, and the composition of the soap liquid, and of course the atmospheric conditions also influence the ways in which the ink is deposited. I didn’t have a chance to interview him last time he was here (my friend’s parents live in Brazil), but I plan to do so on their next visit. Also hope to get some photos of the artistic (and scientific) process, and I’ll post those, plus the interview, on my blog here.
Thanks for that post! I’ve been wondering about coffee stain rings (and why they are rings), but I’d never have guessed they could inspire such cool technology.
Kristi – Soap bubble art sounds very interesting and is a related phenomenon but considerably more complex. In soap bubble art the deposit will also be controlled by the drainage of ink from the thin liquid membranes in the bubble walls. This adds a further set of physical constraints that will interact with the different dyes and pigments in his bubbles or foams. I suspect that the introduction of a new set of variable that control ink supply as well as those that control drying will introduce unpredictability into the end effect, resulting in complex and beautiful art work.
Uncertainty, albeit controlled by well understood physical/chemical principles is also important in raku glazed ceramic ware.
Here, at least in its modern western interpretation, ceramics are glazed using metal oxide pigments that change colour dramatically when exposed to a reducing furnace atmosphere. The example above has used a copper oxide glaze with black cupric oxide, blue-green cuprous oxide and red copper metal all potential final colours. The object is placed in box of combustible material (sawdust, paper etc.) straight from the hot kiln and the resulting conflagration results in a random swirling pattern of gasses that “randomly reduce” the glaze to patches of different colours.
@ Brian – The different ways in which the pigments are deposited by the demise of soap bubbles are very interesting. Sometimes the ink is deposited within the perimeter of the bubble ring, and the resulting image looks like a planet or a marble. Occasionally there are much smaller bubbles caught at the perimeter, and those make their own patterns. Some of the artworks are on display at a gallery here, but I think it will make a more engaging post to see both the images, and the process of the artist at work. I can also include temperature and humidity data for the images, as the artist/scientist keeps track of these variables.
Brian – are you planning to include the lovely molecular biology application of inkjets? Agilent’s microarrays are printed using technology leveraged from Hewlett-Packard’s inkjet research, a fact that occasionally* impresses visitors to our lab.
*with very small values for “occasionally”
@ Brian – I found a link to a preview of the soap bubble art here; if you scroll down to the second entry under “Prêmios”, you can see Rogério with one of his artworks. There’s a link to a more detailed article as well, but it’s in Portuguese (naturally, being on a website for an art gallery in Brazil). Hopefully I’ll get some good photographs of the images, along with my interview … and it will be in English, of course. ;-)
@Kristi – thanks for the link. It looks interesting. I hope to read the interview and see some more images.
@Richard – there is a company in Edinburgh Arrayjet that does it too. HP technology heats up the fluid to generate the drops so I might be worried. My review is on drop generation, impact, spreading and drying – the physics behind the process. That does not leave much room for applications!