Why are coffee stains darker at the edges?
25 comments
·May 11, 2025croemer
mseri
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
Indeed, article appears to be old encyclopedia style, no citations, oversimplified.
null
aaron695
[dead]
ape4
At first I thought this website would be pages with title "Why is..." but the .is is Iceland's TLD ;)
jhaile
It's too bad they don't use more user-friendly URLs like why.is/coffee-stains-darker-at-edges
michalpleban
I am pretty sure they used this domain hack on purpose :)
rolph
[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
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.
nashashmi
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
Diffusion, more specifically capillary flow I think. Water will flow from the saturated to the unsaturated areas.
michalpleban
Because it evaporates [mostly] from the edge, so new water flows there to make up for it.
marcusverus
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.
Fnoord
My guess would be: because there is more space in the outer ring than the inner ring.
vlan0
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
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
Also blood.
Kaibeezy
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
I feel like that's completely unrelated.
neogodless
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.
thisismyswamp
fluid pressure pushes particles outwards
bloqs
Because of neurodivergence causing their perception
rs_rs_rs_rs_rs
Hah! What a great domain name!
Corey_
It’s kind of amazing how a dried coffee ring ends up being a battlefield of fluid dynamics, evaporation gradients, and capillary flow. What we used to wipe off the table without a second thought is now a playground for physicists. Makes you wonder how many other mundane messes are hiding Nobel-worthy math.
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