[HN Gopher] Why are coffee stains darker at the edges?
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Why are coffee stains darker at the edges?
Author : michalpleban
Score : 126 points
Date : 2025-05-11 21:03 UTC (2 days ago)
(HTM) web link (www.why.is)
(TXT) w3m dump (www.why.is)
| rolph wrote:
| [supplementary]
|
| Radial chromatography:
| https://en.wikipedia.org/wiki/Radial_chromatography
|
| when liquid phase is applied to impermeable solid, i.e. glass
| sheet.
|
| you have solid phase "radial" chromatography.
| jampekka wrote:
| This is not the same phenomenon though? Chromatography is based
| on different adsorption affinity of the different molecules of
| the liquid to the stationary phase.
| rolph wrote:
| oh it very much is!
| neogodless wrote:
| This has been a little mystery for me when I don't immediately
| dispose of my pour over coffee filters. Similarly they end up
| quite dark at the edge.
|
| But as per the article, that's where most of the evaporation
| happens, and more of the color is left behind there.
| croemer wrote:
| What the article doesn't emphasize enough: Pinning of the contact
| line is crucial (e.g. due to surface roughness), otherwise the
| ring would not be as pronounced. Due to higher curvature,
| evaporation is faster at the edges, causing the non-evaporating
| solids to flow to the edge leading to more of them there in the
| end when everything has dried up. But on a smooth surface,
| droplets just shrink. When they don't, you get the ring stain.
|
| Relevant: https://www.nature.com/articles/nature10344
| mseri wrote:
| It is a bit annoying that the article does not link any
| relevant research. There is a wikipedia page on the topic
| (https://en.wikipedia.org/wiki/Coffee_ring_effect), but afaik
| it is an interesting problem in many different contexts, for
| example in inkjet printing (one can find plenty of articles
| there as well).
| croemer wrote:
| Indeed, article appears to be old encyclopedia style, no
| citations, oversimplified.
| zengief wrote:
| A better ref imo:
|
| https://www.nature.com/articles/39827 (1997)
| moregrist wrote:
| This is the original seminal work on coffee drop evaporation
| out of Sid Nagel's lab, with theoretical support from Tom
| Witten and Todd Dupont and their students.
|
| Like everything out of the Nagel lab, at least from that era,
| it combines a keen curiosity about things we take for granted
| with rigorous physical experiments and insight.
|
| The Nagel/Witten collaboration was one of the many lovely
| things at the University of Chicago in that era, and it was
| always tremendous fun to see them present and get a glimpse
| at how they approached problems.
|
| It was like looking over the shoulder of giants: often
| humbling and always educational.
| vlan0 wrote:
| Hmm not just coffee stains too. If you've ever had a water leak
| on gypsum board, the edges of the water ring are darker.
| Retr0id wrote:
| I think that's a similar but different effect, as the water
| travels outwards from the centre due to capillary action it
| pulls particles with it.
| cameronh90 wrote:
| Also blood.
| shagie wrote:
| Side bit from some recent news about dried blood and its
| crackle pattern... https://phys.org/news/2025-04-blood-
| droplets-inclined-surfac... (saw it
| https://news.ycombinator.com/item?id=43852446 though it
| didn't get too much attention)
| IAmBroom wrote:
| Also all particulates.
|
| It isn't about the chemistry of the suspended/dissolved
| solids.
| ape4 wrote:
| At first I thought this website would be pages with title "Why
| is..." but the .is is Iceland's TLD ;)
| michalpleban wrote:
| I am pretty sure they used this domain hack on purpose :)
| jhaile wrote:
| It's too bad they don't use more user-friendly URLs like
| why.is/coffee-stains-darker-at-edges
| ForOldHack wrote:
| This is WHY I read HN daily. Omg.this.is.funny.org
| any1 wrote:
| This is exactly what I've done with my blog. See e.g.
| https://andri.yngvason.is/repairing-the-washing-machine.html
| rs_rs_rs_rs_rs wrote:
| Hah! What a great domain name!
| nashashmi wrote:
| Evaporation is more at the edge. More of the water makes its way
| to the edge. The water carries more color to the edge. So that is
| why the ring of coffee color is formed.
|
| But why is the water making its way to the edge all the time?
| gibagger wrote:
| Diffusion, more specifically capillary flow I think. Water will
| flow from the saturated to the unsaturated areas.
| Fnoord wrote:
| My guess would be: because there is more space in the outer
| ring than the inner ring.
| michalpleban wrote:
| Because it evaporates [mostly] from the edge, so new water
| flows there to make up for it.
| marcusverus wrote:
| Gravity / water pressure. Consider an overly simplified
| case[0]: A molecule "disappears" from the edge, leaving a
| cavity (blue circle). Waiting to flow into the cavity are two
| molecules, one on the inner side (red) and another on the outer
| side (purple) of the cavity. Molecule on the inner side is
| being "pushed" into the cavity by a much larger "body" of water
| (pink) than is the molecule on the outer side (light purple).
| So even though both molecules will move into the cavity, the
| inner molecule will move farther. Repeat a few quintillion
| times, and you've got directional flow from the middle to the
| edge.
|
| [0]https://i.imgur.com/mVOiwxH.png
| andrewflnr wrote:
| Because the drop/puddle is trying to keep its shape. I think
| that's what the current top comment is saying about the contact
| line being fixed. On rough surfaces the edge can't just retreat
| as it evaporates, and if I understand correctly it also wants
| to keep the rounded shape at the edge due to surface tension,
| so water gets pulled in from the rest of the puddle to fill it
| out.
| Kaibeezy wrote:
| This is the same reason suburban sprawl continues to grow despite
| the reduced density at the edge. There's a premium for a
| perception of being mostly surrounded by open space, out past all
| the other housing developments and strip malls that are a back
| towards the city. It creates a bump of economic gradient at the
| frontier.
| filcuk wrote:
| I feel like that's completely unrelated.
| ForOldHack wrote:
| Exhibits the same behaviour.
| IAmBroom wrote:
| Coincidentally, ergo unrelatedly.
| Kaibeezy wrote:
| Basically the same curve. Reminded me of it. That's all.
| noboostforyou wrote:
| > There's a premium for a perception of being mostly surrounded
| by open space
|
| Maybe? In urban areas the opposite is true - rent goes up the
| closer you are to a major subway station
|
| https://www.renthop.com/research/nyc-mta-subway-rent-map-202...
| harrall wrote:
| People buy at the frontier because they can afford the housing
| there, even at the severe loss of amenities.
|
| Especially if the next 20+ years of their life is going to be
| driving their kids to sports games anyway.
| bloqs wrote:
| Because of neurodivergence causing their perception
| thisismyswamp wrote:
| fluid pressure pushes particles outwards
| nthingtohide wrote:
| I think this was explained in a documentary by Discovery Channel
| some 20 years ago. I remember it vividly. One application of this
| was to use this process to manufacture very thin wires by
| deposition of atoms.
| logic_node wrote:
| It's because as the coffee dries, the liquid gets pulled to the
| rim, leaving all the coffee gunk behind in a ring. Turns out,
| this same trick helps make better inks and paints too!
| hydrogen7800 wrote:
| I once noticed on a neighbor's garbage can, which had their
| house number spray painted on it, that the paint had mostly
| flaked off except around the edge of the numbers which was
| adhered better. The paint would have been thinner from the
| spray application.
| Apocryphon wrote:
| Serendipitously enough, I _just_ started skimming this book of
| factoids I got from Five Below, and three questions in this
| phenomenon is addressed:
|
| https://www.google.com/books/edition/Do_Geese_Get_Goose_Bump...
| Skunkleton wrote:
| Is this completely correct? Coffee isn't homogeneous. There are
| particulates and oils that will separate out. Anything pushed to
| the top will also move towards the edge given the shape of the
| droplet. There is also capillary action to consider. Seems like
| there should be more than one effect that leads to the edges of
| the stain being darker.
| rdtsc wrote:
| And of course, if you use LaTeX, and need coffee stains on your
| paper there is a package for it:
|
| https://ctan.math.illinois.edu/graphics/pgf/contrib/coffeest...
| kazinator wrote:
| Many kinds of stains are darker at the edges. It's because the
| capillary action slows down there. The area of the stain
| increases in proportion to r^2, and there is evaporation also.
| Thus the stain only spreads to a certain size. As the solvent
| thins out, it's not able to carry the pigment quickly, and so the
| pigment particles pack closer together. Pigment is still arriving
| from the center of the stain, but not moving farther out any
| more, so it has to accumulate.
| saltcured wrote:
| I'm disappointed the article and all the comments here ignored
| Mach banding...
|
| On top of all the mechanism that would distribute the solids in a
| bit of a ring, we also have a perceptual distortion that would
| enhance the contrast a bit, making it look like a stronger
| gradient than it actually is.
| ozten wrote:
| Glancing at the domain name, I got a burst of nostalgia for
| whytheluckystiff.net.
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