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I understand that you get coffee rings on a table as a result of solute migration (solutocapillarity) towards the pinning of the circumference of the coffee ring [Deegan et al.].

Below is an observation that I made with coffee in a mug. I notice several rings forming along the inside circumference of the mug and these rings seem to be equidistant. Two things, at the very least, hit me:

  1. Is the equidistance in the rings only because of a generally constant sip volume?
  2. Is there a penetration depth from the surface where most of the solute collects and migrates in. I am assuming based on brewing observation that coffee grounds (the solute) are less dense than water and float on top.
  3. If the second hypothesis is correct, I suppose the rings would be closer or further away depending on how much coffee I use?
  4. Is there a physics/chemistry coupling to this problem?

Mug ringsin coffee cup

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1 Answer 1

As much as I hate answering my own questions, I can assure you, it is not about the "points"! :P

I performed an experiment. Today (4/1/2013), I made myself another batch of the same coffee. I only eye-ball measured the coarseness of grinds and the amount of coffee beans I used initially. I am quite sure that this measure was well within 5 grams of what I used on 3/31/2013.

Compare photo from 3/31/2013: enter image description here

With photo from 4/1/2013: enter image description here

  1. Both show the presence of equidistant coffee rings on the inner circumference.
  2. Photo from 4/1/2013 shows that these rings die down after a certain depth while the rings are quite persistent in the photo from 3/31/2013. Mind you, I am not entirely sure if the depth was the same in both cases and if a change in the circumference (since my mug is tapered) had any effect on the proceedings.
  3. From both photos, it would seem like one of the editors of this question said, the equidistant rings were probably because of my "sip volume" being quite constant! (which is good observation! I guess I have subconsciously trained myself to sip a certain constant volume!!).
  4. The difference in these photos is that the one from 3/31 shows a ring of foam along the circumference while the one from 4/1 has NO ring of foam.
  5. I know from Deegan's paper mentioned in the question and my own observations of fluid dynamics that pinning at the 'edges' can cause a mass flux towards the pinned edge.. in this case a pinned circumference.
  6. I also know that foams are actually thin liquid films that are holding together air [reference-can't find the actual pdf article]. Foams always have the liquid draining from the foam interface towards their pinned ends.

Based on 5. and 6. I am tempted to say that although the sip volume did play an important part in the depositing the coffee particulate matter uniformly around the circumference, the effect of the foam perhaps cannot be neglected. The presence of the foam allowed for the transport of coffee particulate while the lack of the foam did not allow for this mass transport to occur.

Any more thoughts on this? This is probably open to further interpretation!

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