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).

A better ref imo:

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

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

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

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

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.

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

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.

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.

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

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

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.

Also blood.

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.

I feel like that's completely unrelated.

> 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...