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
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
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