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
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:
Latest Answers