This question popped up in my mind when I was going through the explanations of coffee ring effect. Almost all of the explanations state that “the outer rim of the drop evaporates faster”, but unfortunately I couldn’t find any explanation WHY it dries faster than the inner part. Is it something to do with height? area? surface tension being stronger/weaker on the edge? temperature?
Edit:
Sorry for not adding any image, but what I actually meant was this:
[uneven evaporation of drop](https://www.ds.mpg.de/3208834/original-1559025408.jpg?t=eyJ3aWR0aCI6ODAwLCJoZWlnaHQiOjYwMCwib2JqX2lkIjozMjA4ODM0fQ==–01a65b684acafba312604a11b22490739a711d36)
As you can see from the image, the arrows indicating the magnitude of evaporation are stronger on the edges in 2D cross section. I was wondering why it is stronger there, rather than being even on crest and on the flanks of the drop
In: Chemistry
If you are refering to the observation that the drop seems to shrink in size instead of flattening, it has more to do with surface tension than different evaporation speeds.
Surface tension is what creates the drop shape in the first place. Regardless of where the water is lost to evaporation, the remaning water will continue to group in a drop.
On top of that, unclean surfaces promote drop formation since the water doesn’t adhere that much to the surface. If you do a professional cleaning (way beyond what people understand for cleaning) water can indeed form a thin film over the surface. On this topic I recommend you check this video from Applied Scienc (a true goldmine of a channel) on cleaning, with practical demonstration:
[https://www.youtube.com/watch?v=HiL6uPNlqRw](https://www.youtube.com/watch?v=HiL6uPNlqRw)
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