Why are coffee stains darker at the edges?

https://www.why.is/svar.php?id=5513

by michalpleban • 1 year ago

135 45 1 year ago

why.is

rolph2• 1 year ago

[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• 1 year ago

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.

rolph2• 1 year ago

oh it very much is!

neogodless• 1 year ago

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.

croemer2• 1 year ago

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• 1 year ago

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).

croemer2• 1 year ago

Indeed, article appears to be old encyclopedia style, no citations, oversimplified.

zengief• 1 year ago

A better ref imo:

https://www.nature.com/articles/39827 (1997)

moregrist• 1 year ago

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• 1 year ago

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• 1 year ago

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• 1 year ago

Also blood.

shagie• 1 year ago

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)

IAmBroom2• 1 year ago

Also all particulates.

It isn't about the chemistry of the suspended/dissolved solids.

ape4• 1 year ago

At first I thought this website would be pages with title "Why is..." but the .is is Iceland's TLD ;)

michalpleban2• 1 year ago

I am pretty sure they used this domain hack on purpose :)

jhaile• 1 year ago

It's too bad they don't use more user-friendly URLs like why.is/coffee-stains-darker-at-edges

ForOldHack2• 1 year ago

This is WHY I read HN daily. Omg.this.is.funny.org

any1• 1 year ago

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• 1 year ago

Hah! What a great domain name!

nashashmi• 1 year ago

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• 1 year ago

Diffusion, more specifically capillary flow I think. Water will flow from the saturated to the unsaturated areas.

Fnoord• 1 year ago

My guess would be: because there is more space in the outer ring than the inner ring.

michalpleban2• 1 year ago

Because it evaporates [mostly] from the edge, so new water flows there to make up for it.

marcusverus• 1 year ago

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• 1 year ago

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.

Kaibeezy2• 1 year ago

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• 1 year ago

I feel like that's completely unrelated.

ForOldHack2• 1 year ago

Exhibits the same behaviour.

noboostforyou• 1 year ago

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• 1 year ago

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• 1 year ago

Because of neurodivergence causing their perception

thisismyswamp• 1 year ago

fluid pressure pushes particles outwards

nthingtohide• 1 year ago

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• 1 year ago

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• 1 year ago

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• 1 year ago

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• 1 year ago

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• 1 year ago

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• 1 year ago

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• 1 year ago

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• 1 year ago

Glancing at the domain name, I got a burst of nostalgia for whytheluckystiff.net.

mikhailfranco• 1 year ago

Same for watercolor painting:

https://grail.cs.washington.edu/projects/watercolor/paper_sm...

Why are coffee stains darker at the edges?

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