Let’s say you spill some tomato soup on your counter. It dries. You go to wipe it away after it dries, and the center wipes away easily, but the edges stick and need additional scrubbing to get out. I’ve seen it with other hardened liquids, too.
Why?
In: 993
Edges are more dried. If you let it sit for multiple days to completely dry, it will all be difficult to get up.
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The other answer here isn’t fully right. I think you’re asking about the [coffee ring effect](https://en.wikipedia.org/wiki/Coffee_ring_effect), where even after everything is fully dried, the outer ring has more “stuff” and is therefore harder to scrub away.
Is has to do with how that drying happens. When a liquid with suspended solids (like coffee, soup, milk etc) spills and then evaporates, that evaporation happens from the outside edges of the drop. It pretty much has to evaporate at the surface, right? Well as that happens, there’s a [flow of liquid from the drop interior out towards the edges](https://pbs.twimg.com/media/DNDdSJdWAAAOR2C.jpg), and that flow carries the suspended solids too. By the time the drop fully evaporates, this outward flow has [built up a “ring”](https://onlinelibrary.wiley.com/cms/asset/763784d4-d2ac-453c-9060-75d2e6b40e8e/ppsc201800098-fig-0001-m.jpg) around the outward edges of the spill, leaving less in the middle.
There’s actually been a lot of formal academic study of this effect, when and how it happens, and when it doesn’t.
Some more figures and diagrams:
[https://static-01.hindawi.com/articles/acmp/volume-2018/9795654/figures/9795654.fig.006.svgz](https://static-01.hindawi.com/articles/acmp/volume-2018/9795654/figures/9795654.fig.006.svgz)
[https://ars.els-cdn.com/content/image/1-s2.0-S0001868617303664-gr7.jpg](https://ars.els-cdn.com/content/image/1-s2.0-S0001868617303664-gr7.jpg)
A microscopic video of it actually happening:
Basically, spills aren’t flat
If you look closely, they kinda thin up at the edges, and have a peak near the middle
Since it has to dry from the outside inward, the rings dry up firmly before the centre has even started
Droplets and spills are often kinda dome shaped so the edges are actually thinner than the middle part so they dry up faster and stick more to the surface while the middle stays wet longer.
Fun fact, this effect can be seen when working with radioactive stuff by basically taking a ‘picture of the radioactivity’ — when the water dries up, the radiactive stuff is accumulated at the edges of the droplet [like in this picture (red = more radioactive)](https://journals.aps.org/prc/article/10.1103/PhysRevC.89.064318/figures/4/medium). The people who were doing this studies were able to avoid this by using a surface that doesn’t want to get wet.
~~Fun fact,~~ [~~this effect can be verified by using radioactive solvents and then acquiring radiographic images as shown here~~](http://dx.doi.org/10.1103/PhysRevC.89.064318)~~. This distribution was not desirable, so a lot of work went into getting a uniform covering of radioactive material on the surface,~~ [~~e.g. by using a superhydrophobic surface as backing~~](http://dx.doi.org/10.1016/j.nima.2012.02.024)~~.~~
~~PS: if you can’t access papers because they are behind a paywall, please don’t use Scihub to access them! It could hurt the greedy publishers!~~
Edit: reworded for ELI5 friendliness
I’m gonna go out on a limb here without any research and suggest that maybe it’s because a puddle or drop of liquid is subjected to gravity (as is everything else) as it tries to lay as flat as possible it inherently pushes the minerals to the outer edge. When dry you are left with an outer ring with more minerals than the rest of the former fluid.
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